[MetalMan]

Whatever motor you choose, you should shoot for about 35,000 RPMs loaded at max. throttle input. The 7XL on 16 cells will pass this mark (about 42,000). I am not sure why this is, but 1/8 electric buggies do tend to run best at 30,000-35,000RPMs. Even 14 cells is still about 37,000 RPMs. 12 cells would put you at about 32,000RPMs. Of course, you could always go with a motor like the 8XL and run 14 cells, or a 9XL and run 16 cells. This would greatly improve your runtime.
The 9L and 10L also work very well for 1/8 buggies. Either on 12 cells will do just fine, but the 9L will be faster. Although, you most likely won't get up to 50mph unless you go with something like 16 cells and a 9XL. 50mph really is asking a lot! What are you doing that requires so much speed?