So when he had carbons and regular broadheads then the arrow would plane too much (because of the speed) and he couldn't get it under control.
Hikchick,
One of my good friends, who's bow I tune, shoots 301 fps (Bowtech Black Knight) with fixed blades and his bow shoots great. I wouldn't narrow it down to a speed thing. It's more like a speed/broadhead/tune thing. Some broadheads will shoot very well out of a well tuned bow even at 300 fps.