I am not sure how fast a stock TC3 RTR goes.
The RS4 RTR EVO with the .15 motor is supposed to go about 45 MPH. The RS4 18ss kit with the bigger motor is supposed to go mid 50s and it accelerates very hard. The 18ss should easily break 60 MPH with a gearing change and still accelerate hard. The 18ss motor is strong. I've run mine in organized racing in both ROAR and Outlaw classes and I've yet to see any motor pull away from me on the back straight. I run the stock pipe, stock gearing, stock plug, 20% O'Donnell, and a Motorsaver filter. The only motor I would fear is the O.S. 18 TZ Turbo. That motor is bad ass.
I would highly recommend a kit over any RTR. The belt cars like a Serpent 710, MX-3, or an R40 offer a better CG than the shaft cars like the TC3 and the RS4. The belt cars mount the fuel tank, receiver, and battery pack right in the center of the car. These are fairly heavy parts on the car and having them in the center gives a better CG. The shaft cars have the shaft running down the center so the tank, Rx, and battery are on the sides.
The shaft cars are better for casual parking lot running. The belt cars are better for organized racing on groomed tracks. With that being said, there is no reason why you can’t be fairly successful in organized racing with a shaft car. Most people drive like nuts on the track and they end up crashing and burning. Run a smooth line at a decent pace, keep off the boards, let the crazy fast people pass when you see them coming up from behind, finish the race, and you should be in 3rd or 4th place. With a few mods an RS4 or a TC3 should be pretty good but I would not expect to beat the really good drivers with the Pro level belt cars.
I bought my RS4 18ss because I planned to mostly run it around in parking lots with my Savage. I quickly found out that running a TC in a parking lot was not fun for me. I think that running on a track is much cooler. When running in parking lots you will go through tires at an alarming rate, rocks will send your car airborne, and pebbles seem to work their way up into the transmission gears (thus damaging them). Rubber tires suck big time, foam is the only way to go.