If you're looking strictly for energy efficiency, first step would be to upgrade your trigger to a MOSFET, and upgrade all your wiring to low resistance stuff
That will give you a significant boost
Then if you want to spend a bit more money, get a good motor, or your current motor tuned up.
And then of course switch to LiPo. 7.4v LiPo used with a MOSFET will give you around the same ROF as a 9.6v NiMH mini battery