I agree. I don't think 14.8v would have an adverse effect on the motor, but getting a 14.8v battery to fit may be the issue.
I have 2 types of 9.6v NiMh batteries. One is approx. 1-1/8"x1-1/8"x4" long, the other is 1/2"x2-3/8"x4" long. The 11.1v LiPo is 7/8"x1-1/2"x4-1/8" long. The 12v NiMh is1-1/4"x2"x2-7/8" long.
I should get 2 of the 12v batteries tomorrow (Amazon Prime is QUICK!) and 1 will go into the 3rd Rail E7. I already have checked for space and removed the 9.6v battery (will use for another conversion soon). Should know tomorrow or Wednesday if the12v battery enhances the E7 performance.