Lower voltage just doubles the amp draw and heat, the reason it works so well in 1/12 is that the cars don't weigh much. Nowadays the 21.5's in VTA cars are pretty fast.
All the rules need to be revisited, 13.5 was supposed to be a stock or superstock class but with the turbo equipped speed controls they're two winds worth faster so the class is pretty much busted, same w/ 17.5, which now seem as fast as 13.5's were in sedan.
The other day I heard two of the best ideas for stock/spec racing:
#1 no traction compounds. I know some will say "but the cars will be uncontrollable!", to which I answer true, with the current power packages, but when you limit traction, power comes into line until control is achieved.
#2 solder a fuse of a certain amperage in line with the battery. 15 amps? 30 amps? What better way to govern how much power will be used than an in-line ampreage limiter? This should probably be a re-settable circuit breaker...