[SpEEdyBL]

The 9xl will go the same speed as a 9l both on 12 cells. You just have to gear the 9xl higher to compensate for the kv loss. The higher kt of the 9xl will be able to pull the higher gearing and both the 9l and the 9xl will pull the same amount of amps. The motor that runs closer to 40000 rpm will be the most efficient. In that case, the 9l is better for 12 cells. If you want to run 20 cells later on, the 9xl would still be ok.