Castle Creations motor and ESC combo recommendations for the Traxxas Slash 2WD.

Sidewinder w/1410-3800kv - Stock gearing (23/86)  on 2s = ~35MPH
Copperhead 10 w/1412-3200kv - Stock gearing (23/86) on 3s = ~45MPH
Cobra 10 or Mamba X w/1415-2400kv - 26 pinion/86 spur on 4s = ~50MPH

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.