I haven't had the combine in the feild with the muffler on. I ask my dealer the same question and they didn't think it would affect the power, ie' all the tractors have mufflers.
I am not sure it is just any straight pipe, I beleive i read somewhere that it was a venturi pipe meaning it is narrow in the center to add backpreasure.
All Case IH combines come from the factory with a STRAIGHT pipe. It's the same size all the way through. And why would anyone want to change that, they sound awesome!