The only thing I was thinking is simply conditions and applications. A lot of guys use .20's for indoor winter gaming, simply because of the environment - no wind conditions, clear line of shot, inexpensive ammunition, etc.
I suspect that in indoor CQB environments using .20 and getting hit closer could hurt more than being hit by a .28 in an outdoor environment, simply from contact distances - ie: CQB tending to be closer.
Just anecdotally, I`ve been hit in the pinky (my least favorite place to take a shot) with .20 and .28 at about 15 feet, and I can say without a doubt the .28 had me hopping around and swearing like a mofo, whereas the .20 just stung. From that experience I`ve always thought heavier=hurtier.
Interesting thread, and kudo`s DonP, those are really creative testing methods.