Another option would be to have a taller 5th gear cut.
I have actually talked with a local gear company about doing just that - 10-12% taller to drop cruise 300-360 rpm. Not enough to drop off the torque curve, but enough to be significant.
Better material, micro-polished, nitrided, and cryo-treated for efficiency and strength.
The problem is the cost and minimum order to keep set-up costs realistic. I am not sure if there is enough real demand.
Have a better star shifter and heavier spring produced at the same time - for MUCH improved shifting.
Something to think about.