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.