Which prop to go fast?
I would go for an 11 x 6. This is the best compromise for the OS 46 FX for resonable RPM and reduced engine wear...
Also consider fitting a Master Airscrew as it is lighter than the APC.
Finally, as a general rule , if you want a torquy plane you have to choose big disc small pitch, and for speed you should prefer small disc big pitch....
Good luck