They have the exact same ROF. Mah has nothing to do with the rate of fire on a LiPo.
The physical size of NiMH affects the drain, and the amperage it can supply, that's why you get higher ROF from large batteries.
But LiPo's already supply 100% amperage, so only the voltage changes your ROF.
So a 7.4v 20C LiPo will have the same ROF whether it's 1600mah or 6000mah.