I like the idea of using genetic mods and I dabbled in it back in the 5.4 patches, even though I've never seen it used in AARs (Steve was going to use it for the Chinese in the NATO v Soviets campaign but the People's Republic copped it early on) and I don't personally know anyone else who has used it. The consensus seems to be that you're better off just using terraforming. Someone once distilled it as: there are enough viable bodies in the galaxy to inhabit anyway, and if you need a colony where there's nothing viable use Orb Habs. Why spend research to moderately widen the range of planets you can colonize? Looking into it in 6.2 it seems like Steve tried to give it a buff since now the increments are 10% instead of 5% and cost the same RP, but I think his gravity changes have mostly borked it. For those who've never explored it, in 6.2x after the initial 5000 RP base tech you then get:
- Base Gravity +10%
- Base Gravity -10%
- Base Oxygen Level +10%
- Base Oxygen Level -10%
- Base Temperature +10%
- Base Temperature -10%
- Base Temperature Range +1C
They're all 5000 RP techs and researching one will yield another tech for Base [x] +/-20% and 10,000 RP, and so on until it hits a cap that varies by stream. You can mix and match these eg you could make the base gravity range +20%, so that's 1.2G for humans, with a 90% gravity range that's 2.28G max gravity they'll tolerate (correct me if I'm wrong), and then slap on +10% Base Temp which would tailor them for planets that are hot and high gravity. GM tech can be handy for tailoring populations for special cases: I once stumbled across a planet with a hundred million Duranium at access 1 and tens of millions of all other TNs, at moderate to high accessibility. Unfortunately it was so hostile Venus looks like a paradise... I seem to remember over 50 atmospheres of pressure, which sounds wrong now I write it... does Aurora put out freaks like that? In any case it would have been a good planet for 10,000 mines and siphon off some idle homeworlders. Instead I had to spend twice as much on auto mines. If it had just been a high G world it would be an ideal candidate for GM pop.
There are drawbacks to GM though. For example the aforementioned +20% base grav, +10% base temp would cost: 5000 base tech 5000 +10% temp 5000 +10% gravity 10000 +20% gravity 1500 for the New Species research. Total cost 26500RP. Not huge, but it's certainly not a trifling amount, especially early on. Furthermore, the modifications don't preclude the need to terraform a target anyway: if your target world has an atmosphere with no oxygen you will need to add some. If the pressure is too low you will need to add some gas, probably Nitrogen, maybe a touch of anti GHG to maintain ideal temp. If you're terraforming anyway, why bother investing research when you can just chuck more terraformers at it? Even if it costs more, it will likely be faster, as you don't need to conduct research and then convert populations. 1 GM center being 0.25m conversions per year, unless you build more than the freebies you usually get (I captured them from defeated aliens which is the only reason I bothered experimenting with it) you'll find that constraining, although if you're willing to wait they are still people so they'll naturally grow once you seed the colony with an initial pop. You will not, however, have more idle homeworlders being carted off to the planet by civilians - I'm also unsure how civvies handle modded species. I know that civvies will move aliens around if you create viable colonies for them so presumably the same applies to modded species.
That leads to one of the chief drawbacks: complexity. You can easily have say eight human colonized systems, and then Eridanians with -30% temp range and +10 gravity, Dragons with +50% gravity, Rigellians with +20% temp range +10% gravity, and Antarians with +40% temp range. That's requires a bit more management than just having all human pops that are interchangeable. You might make a colony on a new world and start reducing the temp for human tolerances, then find the Antarians have been shipped on and are falling off the perch once the temp drops too low. You might end up with huge numbers of Eridanians and insufficient humans. It is simpler, if less effective, to not use GM or alien pops for that reason.
Furthermore, even the modifications may not be that much of an advantage. It seems to me from AARs that virtually everyone plays human/sol start. That means your base gravity is 1G so your range is 0.1G to 1.9G. Maxing your tech will let you colonize 0.05G bodies... how often have you said "Rats! That body is just under 0.1G! If only I could colonize 0.07G bodies!"? As Emu said, you find more low G bodies than high G, and Steve's change from 0.3-1.7G to 0.1-1.9G has already made many of those previously unusable worlds into potential class 0.
The other modifications are of less utility because unlike gravity they can be fixed by terraforming. Even a handful of terraformers can add the minimum human required oxygen in a year, and it's rare (or at least it was) to find a planet with excessive oxygen - again, reducing it won't be hard unless there's something crazy like 1 ATM of O2 on the planet. There might be some value in minimum oxygen species, since this would enable a fast, single terraforming module vessel to add the necessary wisp of oxygen to worlds almost instantly. Breathable atmosphere was 2 to col cost back in 5.4x so that made a fair swag of difference to the infrastructure cost. That said, it's still marginal value: high O2 worlds would need to be reduced still, and oxygen can't be more than a certain percentage of the total atmosphere so you can't just stick say 0.005 O2 on a body with no atmosphere and make it breathable. Unless temp was perfect, you would still need to TF the temp. Raising the O2 required seems to be of dubious value, since again, excessively high O2 wasn't something I often found, and when I did it was usually marginally too high. Even if the oxygen is way too high, I the tech caps out at +60% O2, which would be 315,000 RP anyway, and only gets you from 0.300 max O2 (0.200 base) to 0.480 max O2... I think I'd prefer to spend that RP on improving terraforming rate - which since 6.2 is in the same category too, so you're using the same scientists.
Temperature has some value, since total GHG factor is capped, such that some planets cannot be raised up to -10C (minimum for basic 6.2x humans). Again, if you really wanted a colony in every system in a chain, or have some bonanza planet, this is useful for those situations because no amount of terraforming will fix your problem. For base temperature raises though, I am not sure: there was a bug whereby anti-GHG was not capped so you could effectively lower any planet to habitable levels with sufficient anti-GHG, pressure and time being the only constraining factors. If that isn't fixed then the utility of raising base temp seems very marginal. There's also the +1C range tech, which is also interesting because you can somewhat have your cake and eat it, making your new species able to inhabit both hotter and colder worlds.
I think what is sadly missing is a methane/oxygen breathing tech and an atmospheric pressure tech. It would be interesting to be able to colonize methane planets, although, again, you could just terraform them. Of course you'd need infrastructure on the GM world: either it already has methane in the atmosphere and the feedstock are using the infra, or vice versa, so your new creatures don't suffocate. Atmospheric pressure tech is sadly lacking when you find planets you cannot colonize because the pressure is much too high to terraform, or where you cannot get the right temp without exceeding the pressure limits. As it stands I think Steve needs to seriously overhaul GM tech, since gravity seems to be the only one that is really necessary aside from rare temperature cases, oxygen tech seems all but useless (perhaps a randomly generated race with far higher O2/CH4 would find the reduction tech useful), and gravity isn't much use for humans since the 0.1-1.9G range came in. It could be a really interesting avenue but it seems to be largely more cost than it is worth.