Voltage is what sets the maximum speed of your motor.
But if you starve the motor of amperage, the speed will be reduced.
So between an 8.4v NiMH and 7.4v LiPo, the higher voltage NiMH battery should cycle your gun faster. HOWEVER, small NiMH batteries have shit for discharge rate, so they tend to starve the motor of amperage.
It's a bit complicated, but simply said; the 7.4 LiPo has a greater potential to make your gun shoot faster than even a 9.6v or 10.8v NiMH mini battery.
If you find your gun is too slow for you liking on the 7.4, you can just upgrade to a slightly higher speed motor, or lower gear ratio.
Stock, Probably expect around 16-20rps with the 7.4 lipo.
And you can run your gun off a 7.4 lipo *reliably* with no modification. Won't hurt anything.
I can spend 2 minutes and type out all the incredibly amazing advantages of having LiPo over NiMH batteries, or you can just read about it here: