Castle Creations motor and ESC combo recommendations for the Redcat Machete 6s:

Mamba Monster 2 w/1515-2200kv - 17T Pinion on 4s = ~34MPH
Monster X w/1515-2200kv - Stock Gearing (16T Pinion) on 6s = ~49MPH

Note: All speed estimates are with stock tire diameter, high quality batteries and appropriately sized battery connectors.
Always monitor ESC, motor and battery temperatures on new setups or when changing gearing or batteries.