ROF is determined by voltage and amperage.
The motor will draw a certain amount of amperage but it is limited by how much the wires can carry and how much the battery can supply. Stock guns often have I believe 18awg wire which can carry an average max of 10A. Even stock TM motors can draw up to 15A I believe so the limiting factor will be the wire. Small batteries like a 1200mAh one can only usually supply around 12A tops. Better guns will have 16awg wire which can carry around 13A of current. Some people rewire their guns with 14awg wire, I found some 12awg wire online that is as thin as standard 16awg wire, you need a lighter to melt the insulation off since a wire stripper will remove the outer layer of wire strands.
Higher mAh does NOT mean higher ROF. Lower internal resistance in the battery cells means higher amperage supply means higher ROF. The ELITE 2000mAh 4/5A high drain cells I use outperform ELITE 5000mAh SC cells. Higher mAh usually means lower internal resistance because of the larger cell size but not always. Higher amperage supply also means better trigger response as the initial motor start up requires a lot of amperage to provide maximum initial torque. A lower available amperage supply means the motor accelerates slower so the initial shot will have a longer lag than the space between shots on full auto.
Remember, voltage is pushed by the battery, amperage is pulled by the motor.
LiPo are nice but the highest ROF is with a larger voltage. A 12V NiMh will reach around 16V when fully charged. Or you could go for a 14.8V LiPo, or a 24V NiMh pack, etc. etc.