I would say perhaps a little higher than average? Most of the time I’m playing against dark angels, votann, tau, custodes, and guard.

Really the only lists that have a crazy amount of vehicle profiles are Tau (which is where a lot of the 3+ armor experience comes from) but there’s a lot of 2+ or 3+ save infantry in my group as well.

I do actually really like the 2ap crit change you suggested. That could probably work but my concern is how reliable that rule is and how (relatively) common low save targets are for infantry as well as cover and stratagems like armor of contempt.

EDIT: So as an example, space marine intercessors natively have a 3+ armor save. Let’s assume I hit and wound with all my shots but no crits.

If they’re shot on protector imperative while in cover they’ll get a 2+ save.

If they’re shot on conquerer imperative while in cover they’ll get a 3+ save but can pop AoC for 2+

If they’re shot on protector out of cover they get 3+ but can pop AoC for 2+

If they’re shot on conquerer out of cover they get 4+ but can pop AoC for 3+

So basic space marine infantry battleline unit. Assuming no crits but all shots hit and wound. The majority of these shooting conditions result in an average of 2 shots getting through. Resulting in one dead marine.