When I replaced the exhaust (Cat back) on my Syclone(it was all rusted out), it didn't have a muffler, so I just went straight back from the CAT with a 3" pipe. Sounded good.
I think you have to have a muffler if your exhaust is too loud. With stock exhaust from cat to turbo and a 3'' straight pipe out is not loud at all. It was still too quiet for me. (on my Ty)