What happened to Air:fuel ratio as revs went up @WOT? It was rich down low. Did it get excessively rich higher up? People equate throttle position to RPM and those are quite different variables. At wide open throttle, fueling changes as revs rise which is what is known as the fuel slope. On the average older style two stroke the idea is to get slightly richer as revs rise.
On a 4 stroke the air (bleed) jet that feeds air into the needle jet to atomize fuel is used to alter the A:F where on a 2 stroke typically that air jet is missing or is set at 2.0mm and the primary needle jet nozzle height (primary choke height) controls the fuel slope.
And was the throttle wide open at that 6,000 peak? If "yes" to both, try going down on the main jet until it's a little cleaner at 6,000 and see what happens to A:F and power higher up. If it gets even richer, try a shorter needle jet. When A;F is optimized up to say 6,000, try shaving 1mm off at a time to see if you can trim the fuel slope.
If the A:F looks good the whole rev range, try changing the head. The stock head isn't designed for high revs and may be causing a problem with insufficient or excessive turbulence when it starts to come on the power band. It could also be a number of other factors including insufficient transfer time-area. Doug F should be able to advise what HP level his ports should be able to support.
Our calculations suggested that 30 was about right at 6,000 and that it should just be starting to hit its stride on the way to peaking at 8,000 or so.
Development, unfortunately, is a time consuming process of minimizing one adverse factor at a time and then going round that same loop a few times.
You could always try hydrazine. It's a little unstable and unsafe and other people may die, but you have to prepared to make that sacrifice (I hear people say that it's the presidential thing to do)