A 26x10 should be a good prop for the 80 @ approx. 6500

Decrease-Longevity

Increase-fuel consumption

When you say higher RPM's do you mean 8000-9000?
I do not know the torque/HP curves for this engine but going with Higher RPM's would more than likely gain nothing and loose much in all of the above.

The timing/tuning of an engine will determine what RPM range is best for it. Every engine manufacture has their own preferences. It would be nice if they would publish the Torque/HP numbers. That way you would know exactly where you need to be before even running the engine. There would also be less fighting over the irrelevant HP #'s
Here is an example by Zenoah

I Know Dick's out there, he will give you the low down.