Sounds like you actually prefer 40w since 27w just a transition from 20w? So ultimately do you think 40w is better than 27w? Why - I don't have any experience with it yet, that's why I ask.
40 width is almost always the best because of the way attacks are calculated in combat. The stats of a division will be higher the bigger the division is, which means that 20-widths (or 27, or any other width smaller than 40) will not be able to overcome the defense stat of a 40-width division. Likewise, a 40-width infantry division will have a big enough attack stat to overcome the defense stat of a 20-width infantry division despite infantry attack being much weaker than infantry defense per battalion.
Here's a bunch of common division templates to illustrate my points. All stats shown are with max tech in the respective fields, with the Superior Firepower land doctrine fully researched with Integrated Support and Airland Battle.
Also, here's an obligatory statement of
"the specific scenarios where you'll encounter these templates and need to make decisions like these are pretty much only relevant in multiplayer, in single player pretty much anything works but you'll have a much easier game if you build meta-viable division templates." I'm just using this to illustrate my points on combat width and designing templates based around stat benchmarks.
A basic standard meta 10-0 defensive infantry with all infantry tech and superior firepower right-left will have 214 soft attack and 490 defense.
A 7-2 infantry with maxed out support companies will have 323 soft attack and 403 defense.
Meanwhile a 20-0 defensive 40-width infantry will have 364 sift attack and 932(!!) defense. Almost no division in the game will have more than 932 soft attack.
For a more offensive oriented 40-width infantry, the 14-4 division still has 524 soft attack and 746 defense. Not nearly as good defensively, but it has enough soft attack to beat 20-width infantry. Still won't beat any 40-width infantry though, which is why the preferred 40-width infantry is still the 20-0 division, while tanks are used for offense.
The way attacks are calculated, divisions don't add up their soft attacks before comparing to the defense of the defending division, so even multiple 40-width infantry divisions won't be able to get any attacks over the defense stat of a defending 40-width infantry division. The 20-0 also has twenty more organization than the 14-4, meaning it can fight longer, while also being a cheaper division in the first place due to only using infantry equipment and a small amount of artillery and support equipment.
Speaking of tanks...
Here's a basic 15-5 tank division with medium tank 3 fully upgraded and mechanized 3.
Note the lack of support companies other than engineers (terrain boosts are too valuable to pass up) and logistics (saves so much fuel and the supply reduction is vital). Adding support companies to tank divisions reduces their armor value, which can be vital if you're trying to keep above enemy piercing benchmarks.
The same division with full support companies has 6 less armor (90 compared to 84).
Note that both tank divisions have enough piercing to penetrate other versions of the same tank division.
Anti-tank infantry divisions, on the other hand...
Here's a 19-0-2 inf-art-at division. It's basically a defensive 20-0, but with one infantry swapped out and replaced with two AT battalions, and the support companies filled up.
40-widths can afford to add line AT to their divisions when necessary because they have enough organization and defense already, but swapping AT into a 20w division sacrifices too large a percentage of the division's defensive stats.
Note that this division (AT tech is maxed out) has 86 piercing. It's just enough to pierce the 15-5 division with full support companies, but not enough to pierce the 15-5 division that only has engineers and logistics. This is an example of why little things can end up mattering a lot when designing division templates. It still won't pierce heavy tanks, but heavy tanks won't be pierced by any amount of infantry AT.
The following template has maxed out heavy tank 3's, with 129 armor.
In fact, even the medium tank templates above won't pierce the same template but of heavy tanks. Heavy tanks require tank destroyers to pierce.
If you fill out the support companies on the heavy tank division, the armor goes down from 129 to 120, making them much easier to pierce.
You can pierce the latter division using medium tanks simply by swapping out one medium tank for a (maxed out) medium tank destroyer and adding support AT.
However, to get to 129 piercing you have to swap out most of your medium tanks for tank destroyers, which dramatically lowers many important stats (soft attack, breakthrough, organization)just in order to be able to pierce heavy tanks. These divisions do have one huge advantage over heavy tanks though: the production cost. They cost a max of 10216 compared to 21576 for the heavy tanks, meaning you can expect to have twice as many of these MTD divisions if your opponent is going into heavy tanks. Another valid tactic when going up against heavy tanks is simply to outmaneuver them since you are almost guaranteed to have more medium tanks than your opponent has heavy tanks, even if your tanks can't stand up to theirs in direct combat it won't matter if you can run circles around them and encircle them due to heavy tanks' slow speed.
Lastly, here is the most common 20-width tank template, 6 medium tanks and 4 mechanized. 20 width tanks are inferior in every way to 40-width tanks in my opinion, but I'm including them for completeness' sake.
They will take next to no damage from infantry except for dedicated AT infantry, but they have much lower attack. The 352 soft attack will fail to punch over the defense stat of even a 7-2 infantry division, meaning they will take a much longer time to win battles than a 40-width tank division.
20-width infantry divisions have their niches though. They're not all bad. They have twice the organization per combat width compared to 40-width divisions, which gives them a defensive advantage over 40-widths in some strategies. They're also cheaper by quite a bit. They make good garrison divisions for coastlines, where they can defend well enough against even 40-width divisions due to amphibious invasion penalties. 20 widths are also better than 40-widths if your country is limited by manpower or equipment and you need a high enough division count to cover every province along a front but cannot afford to do it with 40-width divisions. But for basic division vs division combat, a 40 width division is superior to an equal width of combined 20-width divisions 100% of the time.
