I think what the guys here were saying about using NiCD's, is they use the "large" version batteries over the smaller NiMH's more commonly used for stock guns.
RC cars is one thing, but I'd put more stock in testimony from airsofter's running 400fps guns and what they recommend, from there years of testing.
My last TM/Systema 375fps M4 ran on an 8.4 2200 large NiCad, the battery would last all day outside firing full auto bursts.
Here's some info I came across:
http://www.servocity.com/html/nicad_...batteries.html
http://www.srbatteries.com/nimh.htm
Hope this helps