Just adding pitch does not necessarily add top speed. There is a limit. The goal is to run pitch just high enough to achieve the top speed that is limited by drag and horsepower, with the diameter that allows the engine to turn that pitch at rpm where it develops good horsepower.

- Decide what rpm you want to run the engine (likely 14-15k on the ground for most hp)
- Estimate how fast you think that aircraft can truly fly in practice
- Get an idea how much pitch you need via this: http://www.rcpro.org/rccalc/PitchSpeed.aspx calculator using your target rpm
- Figure out what diameter it will spin that much pitch at that rpm, then use that as a basis for experimenting.

My best guess is that you will gain nothing with pitches as high as you suggest. As you mention, it is not a speed airframe. Same deal as a car, just because a car could go 150mph in 5th grear at redline does not mean it will, that depends entirely on horsepower and drag. I would be surprised if you can push it too far north of 100, maybe 110 or a hair better. Maybe a 10-8 as CP suggests, maybe 10-9, that's where I would start.

Watch yer flutter.