I don't know about 12th scale, but at my large outdoor asphault track here, we run a WGT class and we run the following together:

13.5 1s lipo
13.5 4c subc

The nimh cars are signifigantly faster, even given the weight difference. I don't know exactly how much faster a 10.5 turn motor would be, but I imagine it would make the two chassis very close in speed... certainly closer than they are now... but alas, we only run 13.5 regardless of battery.

Cpt.