Several years back I had an oiginal design sport/fun fly type airplane. I had picked up a Rossi .40 from another flier and wanted to compare it to my other engines. I made interchangeable motor mounts with it and also a small case Fox .40 I had. On 10-6 props, the plane was slightly faster with the Rossi, but had more climb with the Fox. I flew it most of the time with the Fox. The plane was more nimble and climbed better. Rossis weigh a ton. Personally, I don't think the extra weight is worth the money, but I like light.

Looking at bang for the buck, it's probably going to be hard to beat a Thunder Tiger .46Pro. I would put a Tower .46 muffler on any of them. It'll get you 700-800 rpm just for bolting it on. Costs $15. The new OS .46AX, I have one, is a sweetheart to run and powerful, but $20 more expensive.

For larger sizes, I wouldn't buy a .61. You can get a .75 or .91 built in the same case, same weight so why go small.