props
One example are the props I've used on my Twinstar. It has 2 Irvine 40's ABC BB. The prop that these engines love is a 10 x 6 APC. With the 10 x 6, I can take off and go vertical until I want to stop, but I figure the speed to be average to any sport plane (about 70-80 mph flat and level). Torque is awsome though. I changed to 9 x 9 APC's to see how much speed I could get. I figure speed jumped up to about 100 mph but I lost the unlimited vertical and I lose a lot of speed in the turns. This plane was much more fun with the 10 x 6's.
The design of the plane will have a lot to do with prop and engine selection. Your ultimate selection will determine your speed.
Good luck and happy test flying!