Let me see if I can answer a couple of questions at once.
Normal hobby servos usually work by having their own internal feedback. You are feeding them a Pulse Width Modulated signal (PWM). they use this signal to determine where to position them selves, much like having a built in feedback pot, which in fact some actually have. So, if you try to get feedback from them, you will have to add a device to create/re-create the position feedback signal from inside, that could be a potentiometer (sorry), an encoder or the like. If you are going to use that signal for position control, then you might as well use a motor to drive the "axis of motion" and use a pot to encode the actual position. As you can see, you would be re-inventing the wheel.
The advantage of using a motor to drive an axis is that you can choose the motor size, power and speed to meet the requirements of what you are trying to achieve. As for the go-kart throttle being driven by a servo, the servo may not have the torque you need to move the throttle. I don't know if I would try to add a motor to a throttle connection, but maybe a servo rated to do the work you are asking it to do on the voltage provided by the RC. Remember, the RC only puts out 6-7.2 vdc. That may not be enough to drive the servo the way you have it installed.
Read Gregs responce below. He may have called it correctly. It's always the simple things we overlook.
