For a dual set up for a 2.5i, you will need your local exhaust shop to weld on a exhaust hanger on the side that didn't have a muffler. GT and 2.5i exhaust parts are interchangeable up to the mid pipe because the exhaust headers are different. Don't go too big in diameter as you'll end up losing back pressure. The biggest I'd go in diameter is 2.5 inches, iirc stock is 2.25 inches.