I've tested my 7.4v 20C LiPo against my 9.6v 1600mah NiMH, and they had pretty well the same ROF (~30RPM)
It's important to note that the size of your cells doesn't affect your ROF as it does with NiMH cells. For example, a large NiMH battery will make your gun shoot faster than a mini NiMH battery. But with LiPo batteries, every 7.4v 20C LiPo should give you the same ROF regardless of the size of the pack or the amperage.
This is a lot to take in for a new player, remember that gunsmiths don't WANT to work on broken guns, we'd MUCH rather be upgrading than fixing. And advice is always free
So before switching to LiPo, PLEASE ask a gunsmith for what you need to convert to LiPo, and what you need to be aware of.
Speaking of which, I should send a LiPo noob info sheet to Strelok so he can make a copy-paste cleverly animated informative poster lol