Think of it as a river
voltage is how fast the river is moving
amperage is how big the river is
and resistance is the rocks slowing down the water
NiMH cells don't actually provide all the amperage needed to operate airsoft gun motors, so we up the voltage to get higher ROF. more voltage means the same amount of amperage delivered faster.
It's also why an 8.4v 4200mah battery will fire significantly faster than an 8.4v 1600mah battery. The larger surface area in the cell offers more amperage, and therefore a higher ROF. Same voltage, more amperage.
A LiPo offers all the amperage your motor needs, therefore less voltage is required. A 7.4v 20C 1600mah LiPo will have the same ROF as a 9.6v 1500mah NiMH battery.
Now since the mini LiPo already offers all the amperage you need, it will have the same ROF as a 7.4v 20C 4900mah battery.
And I think the batteries your talking about are lithium iron phosphate, supposed to have higher energy density and be 'safer' lol