I also 100% agree that the loss of top end can be clutch-related. What I still am working on is..no matter what happens to a clutch and it's ratio-ing, it shouldn't prevent the engine from increasing RPM to the limiter...even down hill with the wind at your back...lol. To my knowledge the only things that can do that is:
1. Ignition restriction as in what the limiter does
2. Fuel and/or Air restriction/limitations
3. Aerodynamics- Power -V- air resistance.
Now for mine, here's what I have been thinking about. Most of you know that I have had a tank vent check valve failure and have added an in-line check valve. That has worked very well by the way. But..that OE check valve was stuck open..why? I first though it was just a failed check flap from over filling but what if it's open because it's got dirt and crap in it..it is ported into the dirtiest area on the machine...and I know gas was washing up and out the vent and that will wash stuff straight back down into the tank. And if enough stuff does get into the tank it will collect either on he intake screen or in the fuel filter restricting flow...and lets add that after sitting a while and turning the key on, the fuel pump does take almost twice as long now to pressure-up and shut off as it did new, one would have to think at least my RPM limitations to the 6800 range may well be fuel supply-related, especially sense it runs so well all the way to it. OK, maybe it could also be a failing fuel pump but with only 800 miles on it I would hope not. I will report what I find.