Higher fps only increases range to very very limited effect within our (typically) 490fps limit
Myself and several other well known gun doctors have proven in the field that a higher fps gun with poor compression will be outranged by 350fps of excellent compression.
It's all about compression!
Brit hit the nail on the head. The BB's is never supposed to come into contact with your barrel. Higher quality = better consistency. Better compression = more pressure behind the BB = the BB won't touch the walls of the barrel = more accuracy.
And to back up what he said, the quality of your hop rubber will play a HUGE role on your range.
The peak performance of your rifle can only be as good as the base you build it on. If you buy a cheap receiver, don't expect the upgrade parts to perform as well as the receiver they were designed to fit in!
Case and point; a chinese VSR-10 clone takes significantly more labor, tweaking and maintenance to shoot as well as a marui VSR-10
Watch your force with gas guns though, they don't work like AEG's when it comes to FPS. An AEG with a cylinder ported proportionally to it's barrel length will show very consistent joules of force using any weight of BB. However, a gas gun typically shoots with more joules of force on heavier ammo.
For example, I know that my WE PDW will shoot at 1.34j of force with .20g BBs, but it will shoot at 1.55j with .28g BBs (that's the equivalent of 410fps with .20g)
If your using bastards, you can't go wrong with .28s, especially around the 1.34j range. Amos has upgrade parts for the KJ's if your interested!