To increase ROF you need higher Voltage, so taking another 8.4v won't change anything
Try a 9.6v or better yet a 11.1v LIPO, but make sure you gun can handle that much voltage... And make sure to read carefully about LIPOs before buying one. Tons of info on the forum here.
A different gear set for your gearbox can increase your ROF as well, but will end up being much more expensive
Hope it helps
Originally Posted by Brian McIlmoyle
Real life comparison,
GBBR- bang bang -- Giggle
AEG-- merrrzip merrzip -- meh