I was the one who posted the YS 45 post.
Mine would run on the Ultrathrust pipe, but it always had poor transition, and a very narrow power area.
You are right on the prop size. The only way the engine would work at all is to have the correct prop to let it come up on the pipe. When you finally get it to come on the pipe, it had awesome performance.
It was just too unreliable to use for what I needed it for. If you were racing full bore it would be good, but not very useful in a sport plane.
The YS 45 and the Ultrathrust pipe are both great products, but not when they are used together.

Greg