If you're going with DC, I would reccomend a "JDM" 4-1. It's supposed to go on the '98-up JDM ITR and thus has a 2.5" collector to match the stock 2.5" exhaust on that particular car. If you already have the regular 4-1 by all means use it and just roll with a 2.5" cat (flanged to mate to the 2.25" collector) and 2.5" pipes.