The OEM ratio on the F hotrod bike is 18/46, I went to 42 on mine and close to where you are at, or 70-72 mph at 5500. You worry too much, the engine is made to spin (the cams come into the powerband at around 5500 rpm) and not a car one that dogs around at lower rpm, you can actually damage one doing that. The 18/42 would top out at 125+ if I wanted to go that fast. The rational person has nowhere to utilize that on the higher end making it somewhat worthless. I used 42 because it is the 1100F ratio.
The 750 actually uses something of an overdrive 5th gear, the drop from 4 is too far. Racers use the 900 5th gear to get a better rpm drop for racing, it is closer though and would make your perceived issue worse.