I heard 6-8rwhp max on stock car.
$400 might seem like a lot but an exhaust is alot more and probably picks up alot less.
Figure airbox - 5-10rwhp
Catback 3-5rwhp
headers 8-12rwhp
I hate to say it but those are real world numbers in my mind so way cost and 8rwhp isn't that bad for little hassle.