Exhaust pipe diameter?

[cmoore]

From what I was able to search, (car is not in driveway so cannot check) the stock exhaust pipe diameter post cat is 2.25". I also saw somewhere that the magnaflow exhaust pipe diameter is only 2".
http://www.magnaflow.com/products?partnumber=16528

I know none of these will really give any real performance improvement, but I would think that going to a smaller diameter would actually reduce performance. Sure, maybe it has a bigger muffler (and noisier)?

BTW I do have a rotted muffler and am still contemplating replacing my stock(ish) muffler. But haven't found many decent options, other than $800-1000 options (still with little improvement). Thinking just going stainless walker or something that will last, but not seeing the advantage of anything else.