535i exhaust question

[bimmerred535i]

all legalities aside, I am thinking of removing the cat since it's off the car and getting an exhaust business to put two mandrel bent pipes in. So what size piping, I was guessing 2.25", do you guys think this size is appropriate and why?

I am interested to see if anyone has an answer for you, but I saw this on ebay and thought it was a cool idea.

http://cgi.ebay.com/BMW-High-Flow-Ca...3A1%7C294%3A50