I originally had a Supertigre 40 with an 11x4 prop. The plane would hover and did the most incredible inverted flat spins, the plane would literally not fall. The 11x4 APC prop also provided plenty of speed, IMO. The engine finally failed, it was 16 years old, so I got an OS 46AX and now run a 12.25x3.75 on it. I got a bit more power, but I lost the flat spins that were so nice. The OS is lighter than the ST 40 was so maybe I lost too much nose and tail weight to keep the momentum necessary for the flat spins.

BTW, I tried an APC 10x5 and didn't like it. The 11x4 provided much more torque with no noticable top end speed difference.