Actually, they're generally MORE efficient than NiMH, especially at 7.4v.
You're draining a battery with more Mah less quickly, but it's providing more amperage so you maintain your RoF.
The reason LiPo's are significantly less expensive is due to their manufacturing process, they're just WAY easier to make than NiMH.
If you're going to do a before and after, you'll have to rewire the gun and install your MOSFET first. Then compare NiMH to LiPo back to back.
To give you an idea, on my LMG I was getting about 700mah/box with 9.6v 4200mah NiMH, now around 400mah/box with 11.1v 4900mah LiPo, at the same RoF (triggermasters allow you to slow down your motor speed)