The DC JDM 4-1 is nice, but has a 2.5 inch collector, so you need to get a test pipe/cat to fit it, or just have a new flange welded onto your cat. Either way, it'll work ok with stock exhaust, but, your factory cat is very restrictive, you may want to consider changing it out.

As far as the stock piping, much of it is 2.25 inch, but, it bottle necks bown to 2 or less at certain places in the B-pipe...

Later,
Aj