Well, I don't want to mislead you..
But I ran a 7.4" diameter prop on my DDust..
And if it was too small diameter to fly the plane, I must have missed it.. from launch to out of sight straight up in just a couple seconds..
Confused?? Don't be...RPM on the ground?? yes, that's where I stand with my tach. Just have to have enough experience to know how much your engine/prop will unload in the air and go from there! What is confusing you is the disk loading of the prop...a 7" diameter prop is plenty for a 3.5lb plane.. (it's one of those geometry type things, area of the disk divided by weight of plane..) but don't sweat all that!
The GRRT will not run worth a crap on props other than 7~8" diameter, so all this has been taken care of for you! If guys are flying DDusts with GRRT's, then surely they have props that will work. So if they work for them, maybe they'll work for you!
Does the pitch make that much difference?? Yeah..it's what makes you go fast!! So you have to run a prop with a 7" diameter, so you can sling enough pitch to go fast!! The 11X6 on your trainer and 40 sized engine ain't gonna go very fast..so you drop down to 9X6..engine speeds way up, but you ain't got no pitch!
So you go to a 9X8..now you have some pitch..you will go fast! But...your trainer engine won't turn it as fast as the 11X6..so yo ain't going no faster! Dang! You need a GRRT!
Now just put a 7" diameter x 9" pitch (or so) prop on the GRRT and see what happens!
What it all boils down to is run disk loading as high as you can with as much pitch as your engine will pull.. then you can go fast....as fast as your engine will pull that much pitch..
And a good rule of thumb..if guys are using it, then it might work for you..but a little change in pipe, plug, prop, fuel, might get you out there past them.