I am using a 10.8v NiMH on my G36k. Lasts me all day long, great semi-response and rate of fire. Only prob is the size of the battery, has to be re-wired to the back of the gun (on a G36, even with the large RAS).
Conclusion, if you got a cleanly upgraded gun, go for it. If you are unsure about your shimming, bushings, and grease, get a gun doctor to give you an OK. Also, make sure your chargers support 10.8v or you might have a very long charge time ahead of you.