Borla's exhaust is 2.5". I went from stock to full header back exhaust on mine and I noticed a huge difference. I think you'll notice a difference as well because of how restrictive the stock header is. You'll also notice a difference in sound
You can get parts for your trans to beef it up from a company called Lentech. I think i spelled that right. I think I've read that the autos will hold like 225hp or something in that range from factory.