What about hop up friction? I mean, actual contact with the bb takes place during the hop up process. The heavier the bb, the more hop up you have to put on. The more friction:
1. The greater a drop in fps (wouldn't there be?)
2. The greater the chance of shooting off course. We've all seen the hop up pictures in the manuals. When you add too much hop, the bb goes anywhere and everywhere. I don't know, it just seems like brushing the bb with something physically solid (so not like air) would have some kind of effect.
I know heavier bb's are great at maintaining momentum and staying straight in the wind, but wouldn't hop up friction cancel that out a bit?
I use 0.28's in a 400+ fps gun myself, though, I don't know, I'm not always convinced they're "that much" better. Once I get in the parts to fix it I'll have to test.
"To invent, you need a good imagination and a pile of junk." - Edison