You appear to have at least a Ham's knowledge of antennas (not to say that a Ham's knowledge is trivial), so why don't you try out the modifications you mention and report back to us with the results? Experimental confirmation is where the rubber meets the road (pun intended - RC car, get it?
I fly RC airplanes and helicopters, and before every flight, I have the system turned on and walk away from the model (with someone holding it for safety) with the transmitter antenna collapsed and verify that the operational range is sufficient both with and without the engine or motor running. Electric-powered craft are particularly harsh on RC receivers (although the new breed of brushless motors is minimizing that problem).
My electric-powered helicopter, operating at 72 MHz, has a wire receiver antenna that is wrapped several times around one of the landing skids. It surely violates every rule of antenna efficiency, yet for the distances at which the small heli is operated, the controllable range is fine. The same is probably true for your car example.
One difference with RC cars, though, is that people are usually racing them while standing in close proximity with as many as 8-10 other guys who could be just one channel away. In that case, the bleed-over from a transmitter on an adjacent channel could cause a poorly tuned antenna to be susceptible to interference.
Anyway, I and others on this forum would surely be interested in your results.
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster