The only problem I can forsee is that a lot of rc car controllers have 70/30 controls. You might be able to turn it off if on the transmitter or if you do eventually go to motors and speedo the sabertooth 5a has a setting for it.

The current issue is that if you center the potentiometer or replace with resistors in the normal fashion you will end up going forwards or backwards whilst the throttle is neutral because the neutral position of the servo will be 50/50.