My head's starting to hurt reading this thread. Cudos and appreciation to those who have obviously put a lot of time and thought into their posts. Not being as adept as others at physics/math/etc... I'd opt for the empirical test route. Definitely think this thread should be cleaned and stickied.
Go out to a game.
Load up a couple of heavier bbs in one mag and some lighter ones in another. Put your goggles on, give your AEG and mags to a buddy you trust who won't shoot you in the nuts or throat/teeth.
Pace off a good distance (50yrds?) and have him shoot you, semi auto, center mass. Wear kit or not. See for yourself. Have him switch up the mags and shoot you again. Go a bit further out and have him shoot you some more.
For determining range, keep going until he can't hit you reliably any more (i.e. 6 hits out of 10 or whatever you think is a good average). Have him switch mags and see if the effective range is further or less.
Throughout, don't have him tell you which mag/bb weight he's using (i.e. blind test so you're observations aren't tainted by preconceived notions). Don't switch guns, brands of bbs, hop-up settings, etc... Have him take notes and you do the same.
Thank him for not shooting you in the nuts, then compare your observations with him and report back to the forum. Let us know what you observed.
Personally, I've noticed that, reduced or increased range or not, the cluster/groupings of shots at the outermost extent of the range of my AEGs tend to be tighter with 0.25bbs (vs. 0.20 bbs). Less spread and fewer fliers. To me, that means more hits on target. I think everything else is theoretical, if you can't hit what you're aiming at, a little bit more kinetic energy/momentum/terminal velocity, etc... won't make a bit of difference. I'm of the opinion that, you've got to know your limits and play effectively within them. If someone doesn't seem to feel the first hit or two, give them a burst of full auto and they'll hopefully notice all the BBs bouncing off them. If they are still too "caught up in the moment" to notice your shots, empty the mag at them, swear/bitch/cry (whatever makes you feel better), change out to a fresh mag and keep playing.
If after you make your observations, you want to increase power, increase range, tighten up groupings, increase rate of fire, etc.... search through this forum. Do a thorough search. It's filled with great advice and good people willing to take the time to help you out.
Just my $0.02.
Have fun, it's just a game. But the theory debate is neat.