40D grip with 'AA' battries, slower fps
...with the AA's
I have the 40d and grip. Tried it with Alkaline AA's, and the frame rate was about half, I'd say (3 fps). I didn't time it, just a guess.
Might be faster with NIMH rechargables, but I haven't tried that. The only advantage to using the AA's with the grip is availability. Nice to be able to find batteries just about anywhere if you need them in a pinch.
Acutally, if you get the SterlingTek bats for about $10 a piece, you can get 6 of them for around $60 or so, and have enough juice for around 4000 shots without worrying about rechanges. That should be enough for anyone.
(640K should be enough for anyone... hehe... showing my age)