The transmitter, if set-up correctly, has nothing to do with how fast your motor goes. The transmitter sends out a signal. The receiver in your car decodes that signal and splits it into signals that different components attached to the receiver can decode. It is these other components that complete the tasks that determine if something is completed as desired.

In this case, you are talking about your speed controller. If the transmitter is set-up properly, it will be able to send a signal that can tell the speed controller to go from neutral to full throttle, back to full brake and/or reverse. If the speed controller or transmitter aren't set-up correctly, full power will not be archived by the motor, due to the settings. Some speed controllers have built in programming to limit power to keep from self-destructing.

When purchasing a new transmitter, things to take into consideration are, comfort, features, power. Some transmitters will feel better to you than others. Some transmitters have fewer features than others, and can mean better set-ups and control. Other transmitters are known for putting out tons of power, and rarely ever glitch using crystals.

Your best bet for finding a new transmitter, go shopping at your local track. Ask the people that have different transmitters if you can hold theirs. Feel it's balance and wheel placement. Ask the owner questions about what they like/dis-like about it. Ask if they ever have reception problems. Find out about receiver sizes and make sure that it will fit your application. Find out if they ever switched from another brand/type of transmitter and why. Get to feel as many as possible, and beg nicely if you can take a lap or two to see if the transmitter fits your taste. Purchasing a transmitter should be a once in a while thing, so dropping the cash on a transmitter should be a no-brainer. It is your cockpit and should fit the driver.