The longer run times with lower capacity result from lipo's generally higher discharge capacity. NiMH are generally rated at a max of 10C discharge on good RC cells (2/3A), likely way less with AAs.

I get about 5 mins of runtime in my MI before thermal shutoff But it goes 50mph!!!

My photon (also half-8, himax 2025 5300kv brushless with a 1300mah 15C 3s lipo pack) ran about 15 minutes last night, pretty much all at full throttle. It's geared to about 40mph right now, so it's no slouch either.

The toughest thing about the MI is finding lipo saddle packs to fit (unless you buy an aftermarket chassis that flips the motor to the servo side so you can use stick packs, much like the photon). You have to remove the AA pack retaining pieces to get anything to fit. I clipped off the mounting brackets too, for a little more room. Add some industrial strength velcro and you're set.

The apogee 1000mah 3s 20C lipo packs will fit; they hang off the end of the chassis a little, but with 2 in parallel they provide enough punch to power anything you put in there.

This thread has some pix of both the photon and the MI with the packs mounted:

[link=http://www.rcuniverse.com/forum/m_5842694/tm.htm]http://www.rcuniverse.com/forum/m_5842694/tm.htm[/link]