Will,

I'd say not much, just another 10 mph... So long as my fingers are not at the sticks!

I'm dead curious though whether the engine can approach 3 bhp given a little more help.

Shawn might have some experience with this as he's managed to get his Rossi's to really turn on an 11x9 prop. Smaller and certainly lighter airframe he was using though for racing.

David