How many times do you normally use 5th non-highway? your in ontario, same as me, so i know there are quite a few roads that have 80km.. and im sure you could ride low rpms in 5th doin 80-90 its only a tad slower than highway speeds and for 50km.. you should be set in 4th no?
I think you'd be better off getting the 5th gear instead of swapping out the R&P... IMHO lol