It's a trade off with a fixed pitch prop. Particularly on airplanes with as wide of a speed range as the RV's have.
You could re-pitch to allow you to use full throttle at 8000 and you will really scoot but I think you would be unhappy with the takeoff and climb performance.
My personal preference is for a prop pitch that gives about 2600-2650 rpm with WOT at 12,500 ft. This will give you a pitch for goo all around performance but it will require you to throttle back some at lower altitudes.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")