Most controllers let you reduce "punch" which usually is an amp limiter. As far as actual speed reduction you do it with the EPA on your radio. The reason most people prefer to get the appropriate motor for whatever racing they might be doing is that dialing back throttle EPA tends to alter the powerband of the motor.
Start with an 8.5 Novak or LRP in your 4wd--you will not regret it. Its not overly fast but still has PLENTY of rip. I suppose the equivalent Mamba would be around a 4600 if you want to go that route.