RE: Regarding SP series engines, props and airspeed...
Well, to gain max speed I would say you have to select the prop that gives you the highest allowed rpm. In case of the SP thats 5500rpm. (It has been 6000 rpm earlier, and I flew hours with more than 5500 without any damage).
The highest rpm gives you the highest power output of the engine, higher Reynoldnumber on the blade (thats already an issue with the rpms of the SP) and thus better efficiency. So if you can choose between higher pitch of the prop and lower rpm or lower pitch and higher rpm I would suggest the high rpm.
A prop I can highly recommend is this one [link]http://www.ramoser.de/home_e/varioprop_e/varioprop_e.html[/link]
Its got slim blades, they look good, you can use them for many engines sizes (gas or electrics), and you can select the optimum pitch for your demand and they have 4 blade hubs.
regards
Andi