the collector size for the dc is 2". and yes you can get a 2.5" inch carsound, but you really do need a bigger header. i know that you are getting up there in hp from previous posts and your hp goals, might be time to look into getting a 2.5" header,cat,and exhaust so each are sized correctly. with as money as you have spent, the exhaust system would be robbing you of some hp which really sucks. btw what size is the greddy evo? 2 3/8?
first off, a collector is a part of a header where the exhaust gases from all 4 cylinders dump into before they reach the cat. a cat has an inlet and an outlet - no collector.
that being said, with your dc header which has a 2" OD collector, getting a 2.5" ID cat as opposed to a 2.25" ID won't do too much good with your current set up.
you're going from 2" OD header -> 1.8" ID cat -> 60mm OD exhaust right now. even if you get a 2.5" ID cat to replace your stock one, your exhaust system would be hindered by the 2" ID dc sports header and the 60mm OD exhaust you have right now. if you're really set on making a lot of power, you should look into updating your header and exhaust as well.
there was a good thread on 2.5" header and exhaust started by surferX a while back. i believe it was called something like "for real power, 2.5 inch exhaust." or something along that line. run a search and read up.
Where can you find a 2.5 inch inner diameter exhaust. I want an exhaust system that'll create an anti-reversion/stepped process but can't find the right header/cat/exhaust combo to do it. Any suggestions?
The only 2.5" cat-back exhaust systems that I am aware of are:
I can't remember if Hytech and SMSP are 2.5" on the ID or the OD. The T1R is 2.5" on the OD. The Kakimoto is 70mm on the OD, which translates to roughly 2.77" on the OD. Kteller's cat-back systems are 2.5" on the OD.