Your #s are in tune to what I have been getting.
On 2001-06-11 14:24, Silver Bullet spewed forth this drivel:
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
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.