Headers for D16?

[will13]

2.25" is fairly big. 2" would be better. I think about 3" for a cat would be a good fit, but once it hits the cat-back it'll depend on the size of the piping anyway.

So should the cat be bigger than the pipe? Is there a rule of thumb for cat sizes vs pipe size?

I'm about to grab some n*power ebay headers although im not sure about the outlet size on them. I doub't il be changing my pipe size.