The age old cell question?

[Batfish]

It's not exactly the situation you mentioned, but at least similar - I've been using the MGM 12012/8L with 8 cells in my brushless MGT. It gets to the low 20s with the stock tranny and 16/52 gearing.
I was actually watching temps and current while playing in my back yard some yesterday morning - the speed control got up to 120F and the motor up to 135F. Ambient temperature was 72F. Running through the grass and putting some resistance on the truck with full throttle, the highest the amp draw went was 66 amps. At full throttle in a straight line it was between 17 and 25 amps.
I'll be putting in the forward-only conversion for the MGT (thanks, Mike) and probably trying a couple smaller spur gears as well.

All that said, I would personally think it's perfectly viable to use the 8L with 8 cells in a racing situation. I would definitely say that you'd want to keep it to the 5 minute races unless you do a lot of experimenting with gearing and try lipo for a 15 minute race.
On paper, it seems that it's always more advantageous to work with more cells, but I can tell you that my setup is working in reality :)