So, I recently went to a muffler shop to look and see how much it was going to be to have a 3inch exhaust with a 2.5inch Y pipe. Needless to say, they tried ripping me off heavily, and I decided with a trubendz kit which is on the way.
But I also had them take a look at my cat, which wasn't more than them hitting it and it made rattling noises inside of it. They said it was busted. For about a month or two, on a cold start and idling, it would make a rattling noise, and at 1500-2000RPMs warmed up, it rattled as well. I figured it was the heat shield, but they said other wise.
So considering I'm putting on a full catback, I've decided to just go ahead and replace it anyways. I need a little help deciding on what to do though.

Should I go with a stock OEM cat, or try and match the diameter of the new pipe with a 3 inch in/out cat? The issue with this, though, is that I'm not sure of the diameter of the DC Sports down pipe, and how that would even be fitted on. Do they make 2.25 (assuming that's what the size of the downpipe is) to 3 inch cats? Does it even matter that much, or is it actually better to stay with a stock diameter?
Not to sure what to do here as bigger doesn't always mean better, but wouldn't it flow better if matched with the rest of the catback?

^
If you plan on forced induction, in the future, then it would be prudent to invest in a freer flowing cat now. This will prevent you from 1) having a 'bottle neck' with an OEM sized cat and 2) having to revisit it again to fix.

I ran the OEM stock cat, for the longest time, with my S/C'er set-up. I had the CT-E headers, OEM sized cat., CT-E B-pipe and axle back.

Then XLR8 offered a 'free flow' cat, back in the day, like the one on the link below.
Not sure if they are still available, though.
https://www.heeltoeauto.com/xlr8-per...a-cl-cl-s.html

It made a difference because I noticed that I could not get close to the HBP 6psi I was observing prior to changing the cat. No matter how hard I ran the engine. So, I could only conclude that the cat opened it up enough to lose that 1+ psi to settle around 4.5-5 now.

I do plan on getting my hands on a Comptech S/C if I can scrape up the money for one. Then all that will be left is finding one for sale (Here's hoping I can get to yours in time!)
I was not aware of this cat, thanks for posting it. I guess I'll try and obtain one of these if they still have them. HeelToe normally puts a 'discontinued' label though, so I might be in luck. Now all I need to know is if it will bolt up to the DC Sports downpipe.
One more question; though my cat is busted, will I still pass emissions? I've seen other threads saying so, but I don't want to risk it. If it will, then I will hold off on the cat a little bit longer.

If the Hi flow cat above is still available, it should bolt up to your DC headers. The O2 sensor wires may need lengthening.
This one bolted to my CL-S6 CT-E headers.
The DC sports downpipe flange size & cat flange size / orientation, of this cat, should be the same.
If there is a difference, it would only be in the circumference diameter of the downpipe size after the flex; otherwise, the flange to the cat should work, logically.
If the bolt holes are a different size, they can be drilled to fit new hardware, if needed.