Well you see.. 30 hp gain is very optimistic. When dealing with hp gains, i would rather be negative, as to not be disappointed when i dyno. In dealing with exhausts, make sure you get the WHOLE thing. yes, this is a lot of money, but the only way to get a lot of boost from your exhaust is to do the whole thing. This means, highflow headers, to your downpipe, to your cat, to your piping and finally to your muffler. Make sure, all parts mentioned are replaced, or you will have a tight spot in your exhaust system. Lets just say you have everything replaced except for your headers. All the highflow crap you put in cant work at peak use. Its being cut short by the stock headers. So make sure, you replace it all. And full exhaust HP gains are only usually 10%. So on a GT, you may gain 15 HP. This is a good chunk. It doesnt seem like much now, but lets just say, you normally aspirated up the ass hole. And you now have 300 or 350 HP. Without that exhaust, you'd sure be limiting yourself. Rather than say, 350HP, it could cut you down to 300, or maybe lower, EVEN if a section of your exhaust system isnt highflow. Its all very important.