The cat in my 2004 wrx probably needs to be replaced and I can get a used (under 1000 miles) 2005 sti downpipe for a lot less than the $800+ for a new OE one.
I have another thread on here explaining all that's going on. Don't really want to mod/tune it right now. Just want to get the P0420 stuff taken care of and see if there's any other underlying issues. So far we've narrowed it down to either the pre-cat or down pipe cat. Car is 9 years old w/137000 miles on it so I don't mind using lightly used parts. Someone offered a used 05 sti downpipe with less than 1000 miles on it, just don't know if it will fit.
Again I'll play the catless card. Its cheaper and a tune should still throw codes for other underlining problems, minus the cat problems. Look at your other thread, flashing a cheap XPT tune with a cheap vagcom cable isn't hard. We'll walk you through it man.