Actually, I believe that there are gearsets with taller 5th gear available for racing use for the 5 speed transmissions, though one may have to replace the entire gear cluster at the same time, and the 1st, second, and third gears are also a bit taller than the non-racing version. For whatever reason, 4th gear is the same ratio in both.
This is not a common or inexpensive change though, and you'd likely be talking with folks like Tom Cutter or Anton Largiader to do the job.
While it would reduce your acceleration ability across the board, you could probably simply swap the final drive from an R80 (look for 37/11 ratio for decent balance between making your gearing taller without going alot taller) without too much trouble, and lower RPMs on the interstate. Just plan on downshifting for brisk overtaking on the freeway.
You will need to get a matching speedometer, or pay ~$200+ to re-gear and calibrate your speedometer to match the new final drive. But, I think the 37/11 ratio was common on the R80RT through the 80s, so used speedos that match shouldn't be super-hard to find.
