RE: New Giant GP Super Sportster!!!!!
After having blown both my ailerons off on an early test flight, I'm shy of pushing the speed envelope much. That said though, mine is currently powered with a SYSSA 30 running an APC 18x6w. It's turning 8500 easily on the ground, and unloading well beyond that in the air in straight and level flight. The plane is moving along at a pretty good clip at this point - much faster than it ever was with the Quadra 42 power. Could be it just seems like that because the engine is unloading though? Maybe someday my Aurora 9 will be able to tell me how fast on a downlink so I can see for sure.
Until then I would think you'll be pretty safe with a 6" pitch prop - as long as your down lines were managed somewhat?