that's just an example, i don't know the numbers... but yes. the 150 vs the 200 cannon load in the 109's does make the nose heavier, and is modeled. HTC mentioned it when they put out the second ammo load.
also, try the 190A5 with the 4 cannon vs the 2 cannon load, definate performance difference. not a ton, but up at high alts, the one with mroe guns can not climb with the one with less guns. i have definate proof of that from a special event where i accidently took the wrong ammo load. at 28,000 i could not keep up with the other planes in the squad, i ran out of WEP keeping up, then it was over, i could not get as high (about 33,000 the capped out at, and i never got above 31,000) and they were faster level then.
i still say "dumping" the little bb guns but firing them off does not change the weight or performance of the plane, if it did, then offline you'd never be able to fly with the 10,000 round ammo load. the nose of the plane would be so heavy the whole CG of the plane would be off.