I'm running a 7XL in a buggy with a MM controller with 14/46 gearing. With a Polyquest 4400mAh pack I'm getting at least 15 minutes of runtime in a track environment. For the track, this setup is crazy fast. If I were to do it again, I'd opt for the 8XL for something not so crazy and more runtime.

As for battery selection, big capacity is great, but comes with a tradeoff. It adds weight. Lots of guys run the 8Ah packs, but I chose to run with smaller packs. I just got the 4400 to try it out and am waiting for a pair of 4S 6Ah packs. I felt that the 6Ah packs were a good trade off between runtime and weight.

My 7XL setup, easily does 40 mph. I think that the 8XL will come close to that and should be a better choice. For 4S, I think that the MM is also hard to beat, due to it's user friendliness. If you have access to a computer to program it, that is.