you can colonize with a void dweller and force growth of the -10% race rather than the -60% version which gives you a total penalty of only -30%
Everyone has those kinds of penalties. Except void dweller in habitats.
Sure, if they settle Tomb Worlds. But guaranteed planets aren't Tomb Worlds, they're 80% habitable (+/- a bit for modifiers). To a void dweller, every planet (except Gaia, Ecu, and Relic) is as bad as a Tomb World
even before the Void Dweller trait in particular affects anything. So no, you can't just colonize with void dwellers and reduce the growth penalty to only 30%.
If you are running slavery then you'll have like +80% or so alloy production
Hmm, checking the numbers at game start for the optimal, maximized case:
+15% Void Dweller, +10% Slaver Guilds, +10% Fanatic Authoritarian, +10% Slave Optimizations, +10% Iron Fist, +2% governor level.
That's +57%, of which 40% depends on your metallurgists being Indentured Laborers. 100% stability would bring it up to +87% but a more realistic (and still good, for early game in a slaver empire) 75% stability gives an additional 15%, for a total of 73%. Call it ~75% because it doesn't take long to level the governor once. It's also worth noting that every part of that except the Void Dweller trait is available to other origins (and some origins, like Prosperous Unification or Life-Seeded, get a similar-sized production bonus from the planet itself and a happiness bonus that gives a bit more stability).
So Void Dwellers get 15% (of base) more production than other non-PU non-LS empires but also need to pay alloy upkeep on their habitats (partially offset by the generator on the "homeworld"). Given how low base alloy production is, they're definitely not winning there until their population grows significantly. They start with lower total population than a planetary Megacorp (even if they are a megacorp themselves, last I checked) or Prosperous Unification, though faster population growth than even a Mechanist or Hive until the non-Void-Dwellers expand. Non-VD can expand twice much earlier than VD can expand once unless guaranteed worlds were turned off and possibly even then unless running very low habitable planets - you're just flat out not going to be able to even start building a new habitat, much less finish one, before the planet-dwellers have their colonies settled assuming half-competent exploration - and that's assuming you reserve no alloys for ships or stations AND you don't bother with staying under sprawl (which is a valid tactic for everybody; starting with bureaucrats is stupid).
Interactions with your neighbors are the real gamble, and depend on both how crowded of a galaxy you generate, how far away people spawn, and what kinds of neighbors you get. An expansion/boom focused VD will presumably retrofit and then scuttle their starting ships and not build any more; this is optimal for alloy production but makes you a sitting duck to anybody who can muster even a basic early-game fleet of 15 or so t1 corvettes (which cost exactly the same as a basic habitat) to knock over your starbase. A friendly neighbor who is willing to Guarantee you rather than pluck you like a ripe fruit could make things very easy, but anything else is a significant risk whereas an empire using those alloys for ships would just go conquer their planets, possibly including homeworld, and have both more population AND more population growth than the VD. Obviously this also depends on what you're playing against; normal AI difficulty (or GA but scaling) is a cakewalk even if you're a spiritualist Lithoid or something similarly stupid, but Starnet normal or GA no scaling will be more challenging (peaking at different times) and Starnet GA non-scaling will be a disaster for the build you describe.
If left alone for the first 20 years, Void Dwellers might conceivably have more colonies at the end of that time than a normal empire if the normal empire got really unlucky with habitable worlds, but even at 0.25x habitable it'd probably come out about a wash (though the VD might have more total population by then since they got a head start on growth... but also might not, against a PU or Mechanist or Hive or even lithoid using Calamitous Birth and digging up the hibernating ones).