to add to the questions above: are you planning on just a few bolt ons and leaving it at that, or are you going to do a blower at some point? if you are going to get a blower then do the 3" to remove as much restriction as possible. if you are staying NA then you will want the 2.5"
theres a lot of guys over 400rwhp that switched back to 2.5" because the 3" was losing power. Make sure you do your research before you pony up the c-notes
3" is a lot of pipe, even for supercharged cars. Unless you're putting out MUCHO horsepower, you're fine with a 2.5" cat back. The engine needs a little backpressure to operate correctly. Go with the 2.5" and save yourself from losing some ponies and some cash
~A.C.