Another option would be to swap the motor out to one with 30% (or proportionally) less hp. That would allow the motor to run at higher throttle/load plus it would weigh less and use less fuel at idle compared to higher gears which only change the throttle/load. I guess it would cost more but it would net more gains.
If I haven't already introduced myself my name is matt and I am a lurker on here and GS. I am an aero and EV enthusist from Australia. You can view my
gas log here (in annoying American units) and you might want to check out the
EV/Aero wiki that I have done a bunch of work on.