I researched going with the larger carb a while back for this same engine. What I found is that the carb gives higher rpm on the ground but in flight doesnt give any drastic performance boost. I am not an engine guru and cant explain why this is so but that is what I came up with after hours of reading through post and PMing other RCers.

On my ST90 I put a jett muffler (similar to the ultrathrust) on it. I went with the 120 sized muffler because I was after more torque rather than top speed. It gave a power boost but not a huge one. The plane really sings on a 13X6APC now but is a little too fast, I was really looking for something to boost my performance on a 14X6. Those mufflers are pretty pricey too.

I am looking into the OS 120 now. Weight wise its almost equal to the ST90 and has gobs more power. If my 120 jett muffler will fit it I think I going to go for it.