I believe 2.5 inlet/out is about standard for the SE-R catalytic converters. I just purchased one from www.fulllineexhaust.com (click on their online store hyperlink). They are the cheapest I've seen so far, for exhaust parts. They carry direct fit cats for your car and they also carry aftermarket Hi-Flow cats. I bought the universal Magnaflow/CarSound 94100 w/02 Bung cat for $66.00 (free shipping). This cat normally sells for like $85.00 to $90.00 online and a local shop wants like $160.00. Either is cheaper than a OEM cat that goes for right around $600.00? from the dealer. I'm having the flanges welded on before it's installed. 
I hope this helps.
I hope this helps.
