Sorry for not being comprehensive. Yes you can get special servos with an extra wire for position feedback. The vast majority of hobby servos don't have that. I have never had a feedback servo and I wonder what benefit they offer in practice.
It seems to imply that the servo is undersized for its load. If it had enough power it would just move to the commanded position.
It seems that the extra wire just makes the internal potentiometer voltage available to the microprocessor. You could probably modify any servo to do that. I wonder how the external microprocessor could make better use of that data than the internal circuitry - especially if you get a regular digital servo.
What I said earler about the risk of breaking the servo's internal gears still applies.
...R