Yep.. this is difficult to answer... and if i recomended a prop there is no garentee you'd be happy. You see speed isn't really what you're looking for...Thrust is what should be chasing. Having said that you've got to understand that these scale type aerobactics are performed at a relitive slow airspeed. 3d(ish) aerobatics even slower ie stalled. This is difficult for people to get a grip of...it's a kinda different mind set.
To achieve this one needs the largest diameter/lowest pitch prop the airframe/engine can handle. the problems you face with that aeroplane is size... small aeroplanes need to be flown "faster" than large aeroplanes so this is something to consider. Again, one isn't looking for max RPM. instead proping the engine so the RPM drop into the higher torque engine band. This is achieved by over proping the motor ie getting the largest diameter/lowest pitched prop that'll suit the airframe. if the airframe/ground clearence is small one can't fit a large enough prop so one has no option other than increasing the pitch, thus again lowering the RPM, then again increasing the speed.hummmm

You're hanging around an ok area but you need to experiment with diffents sizes and get used to their effect. Also one must remember that your flying style needs to change with every type of aeroplane...
As a tip I'd be looking for an even larger prop than a 12" It's been many years since I flew that aeroplane... but I seem to remember a decent ground clearence. Stay with the 12incher for a while and try and get the hand of flying at a reduced speed.

Ok Ive stuck my foot into what was intended to be a simple answer...so I need to go on. The science of prop choice is kind of a Holly grail thing. In competition I'd be carrying several very similar but slightly different versions of the same prop... I'd be changing them as the wing force increased or decreased so you mustn't expect to find a prop and stick with it through think and thin. You have to change as the conditions or even the sequence changed.
I see from you question that you've started getting picky...welcome to the "is this a 15.5 x 14 day or should a stay with a 15.5x 14.5?" Club.

Didn't really answer your question did it...er infact what was the original question?