Number one thing to do, realise all AEGs spread their shots tight bore or not, getting that super perfect stream is a dream(Hey it rhymes and everything)
Regular cleaning of your barrel will help too.
If your wanting to chuck in a higher voltage battery make sure your internals can take it, it will put alot more strain on your internals and can bring about some impressive failures.
Softer hop rubber will give better friction to the BB increasing spin but will wear out faster.
Make sure your not having to dial in tonnes of hop to get the BB traveling like you want it or chances are you will rip the rubber or just suffer jam after jam (Oh and if it does jam get off the bloody trigger!)
Don't bother messing about with barrel length, the S-System is about bang on when it comes to finding the right length. Popular belief says its around 360mm ish I think but I run an m4 with 10cm barrel so hey ho
Dont use crappy BBs, cheap is not cheerful in this situation. Also consider moving up to .25s as they would be less effected by wind and team mates farting but they will travel to the target slower (Dont try cheat the chrono now... we will bust you

)
For me though, getting a high ROF isn't as important as getting a rifle accurate... I would rather hit with five shots than miss with fifty
