At steady state where everything is constant the cap will most probably make no difference, but a car in a circuit will always have different current demands... and here is where the cap will help, because of the high internal resistance of the NiCad or NiMH cells....
About the sweet spot, it is not just about the ESR which is generally the lower the better. But its value is the important one.... too small will not be sufficient, and too large will actually be counter productive. I think unfortunately the sweet spot value is very dependent on the many variables, motor , car, circuit power band used etc.... so it should be taylored accordingly.
The main problem of the power source be it NiCad or NiMH cells are the fact that they have high internal resistance....... just try and use a lead-acid battery in place of the NiCd and see the difference!!! Lead Acid can give absolutely huge instantaneous currents which can fry almost anything.... it is just too bad they are heavy and can spill!!!
