For a TT-01 don't worry about upgrading the servo unless you are putting down serious power.
The radio frequency doesn't make the car faster or slower in response by itself. The modulation makes a big difference for interference. With AM, you will get interference and twitching from all slorts of things around you, with FM hardly any interference (I get zero and race next to a highway and airport), and 2.4Ghz (DSM) is almost faultless.
If you are doing fine with the 27AM radio, keep using it. If you get a high power brushed motor or anything brushless, you will need either FM or DSM, the AM will play up bad.