I know someone that strapped 2 LRP 1800 Lipo's in his GenX, and I think he used a 13.5 motor to test if he could drive LIPO in a 12th scale.
The car's handling was very well and he had about the same power as a good 19T (I saw this myself).
The lower weight was not a problem at all....

I think the only mod he did to the car was raising the center shock somewhat to get clearence over the Lipo's.