The issue with using HV servos in EP cars (when running off your main lipo) is degradation of performance over the run.
For a typical 6min run, you start at 8.4V and finish at 7.4-7.6V - it's not much, but the drop in servo performance means you would be constantly micro adjusting your lines. Hardly ideal for consistency.
A stable BEC voltage is much better, be it 6, 6.6 or 7.4v.
Besides the speed and torque specs of modern LP servos are more than adequate for rubber tyres.