I agree with Disston about what you are seeing. BUT, since you are already in there and considering the taller 5th option, now is the time. I wish I had done that when I rebuilt and installed a replacement gear box in my ST. Of course the ST has the 37/11 FD and just really doesn't like sustained Interstate speeds when I take it on trips (tends to increase blowby, thus oiling the left carb a lot, fouling that plug after two days of slogging the Interstate in warm weather, and using slightly more oil).
And the shiny flakes mean its time to do all the bearings - although from what I've read and seen in my own gearboxes, the one roller bearing up at the front may still be good. I didn't disturb that one in my rebuild and continue to have trouble free operation.
But I do bemoan the fact that I could have installed the higher 5th while I had it all opened up.