b5 1.8t exhaust
 

I was thinking about running a straight 2.5" pipe down, cat back, to a magnaflow 14816 muffler. Is this a good idea? Will any check engine lights or the sort come on? Horsepower gains?

thats what i plan on doing but to a Magnaflow 14862 muffler.. you shouldnt have any lights as long as the o2s are in the cat and connected. you might get a little hp increase. i recommend getting a down pipe as well stock is 2" so if u go 2.5 you will open a lot more for better flow and louder sound.