the stock ls exhaust size is almost irrelevant. the relevant question is the donut gasket that goes b/n the cat and header if that is indeed a proper header for your motor. Old size means diddly it's a matter of the cat, donut gasket and new header matching up and sealing. Now as for gains, they will be minimal(but noticable)and if i'm not mistaken in the midrange area(don't quote that though)