Check the exhaust thread in the FAQ/DIY section, they have pictures that show correct pipe sizes. I think it is 3" on both ends, but, not 100% sure on that.

 

Thanx i've been looking for pics etc. For my 2001 you're correct. It's 3" in/out that necks down to 2.5 into muffler.

 

Finally had a good enough weather day. Cut the cat off and just replaced it with pipe. That definitely was the issue. Runs whole lot better now. Little louder sans cat but I wouldn't consider it loud. Now I wait for spring to tackle the plenum job.