I remember it being posted here on ASC at one time someone went into depth with the physics and found that BB's, regardless of weight, are not stable past 380fps or something like that, like Stalker said.
Don't know about your MP5 Stalker but my ICS gets much better range and slightly better accuracy shooting 400fps as opposed to 350fps (chrony with .20's, use .28's, so 320fps and 280fps with .28's respectively.) But then there is the issue of too much hopup adding too much back spin which can throw the BB off.
From my experience:
350fps M4A1 - best range with .25's
400fps M4A1 - best range with .28's
430fps M14 - best range with .28's
Don't know about you people but I shoot at people in the brush more often than people in the open. .28's penetrate brush much better than lighter BB's so can really reach deep into shrubs and such to hit someone. Then there is also the consideration of people seeing the BB flying at them and dodging it. Several people I have seen at games have barely been hit then asked the person who shot them what fps and weight of BB they where using, more often its 400fps and .25's. I have also talked to several people and experienced it first hand where I barely dodge a BB well within the maximum effective range of someones gun and it turns out it was a .28, if it had been a .25 it would have hit because its moving 20-30fps faster.