very little.. removing the second set of cats and adding a muffler with better flow (aftermarket ones probably work better for that) adds a little. also, by doing that you shed some weight which is great. not with the Fister thought.. i think its the same weight as the untouched OEM, it felt heavy in my hands.
i probably shed around 40lbs when i removed the cats and had this light muffler from Top Speed. car felt a little bit quicker but not by much.
if you want to add noticeable power with exhaust mods you have to get sport headers and tune the ECU otherwise it all seems negligible. Porsche is known for putting restrictive headers on the Boxster to keep the power down, we don't want it to be a 911 now..
most of us have done it for the sound more than anything. if you get it right it really transforms the car for the better but i will say that i am just about DONE with exhausts and rusty bolts!!
