A 10x5 3-bladed prop would be fine on an Avistar. You could even try the Evolution TPS 3-bladed prop if you'd like, I believe it's a 10x4 3-blade and should be readily available in most hobby stores.

Less efficient is a good thing when you're not trying to run at top speed. I'm running an APC 11x4 2-blade on my Tower Trainer just to keep the power in check coming from my .46 FX. If I put an 11x7 on that engine, I'd bet that trainer could fly 80 mph.