I use an 11-4 on a Jerry's Little Boy and on a Big Lazy Bee, both with an OS.40. Both are very slow, fun-fly planes. I think an Avistar would be happier with a 10-6. An 11-4 will accelerate you down the runway faster, and probably climb more steeply, but I don't think you'l ever get to what feels like "cruise speed".

It only costs 3 bucks to try, tho'.