Adding 1 5/8" equal length shortie headers on a stock car is a really quick way to add another tenth or two onto your 1/4 mile time.
The velocity is a byproduct of the real culprit; the tuning effect has much more to do with it.
The headers are actually designed to help pull exhuast gasses out of the engine while the exhaust valve is open. This will lower your cylinder pressure. Cylinder pressure is directly related to torque production, and at low rpms the overlap time is greater, allowing for more exhaust gas to leave the cylinder, resulting in lower torque production, and a loss of power until higher rpms.
I would seriously doubt the validity of ANY dyno run showing an 8hp (peak) increase in hp production with equal length shortie headers on a totally stock engine.
Adding a high flow H pipe and freer flowing exhaust compounds the problem on a stock engine.
Full length headers are designed to help create a tuning effect at higher rpm's, yet they also have a tendancy to create a backpressure pulse to keep some backpressure on the cylinders so they do not rob as much (or any) torque. Shorties simply cannot do that, their collectors make it much harder to create that type of backpresure.
So while the headers may increase hp production by 8hp, it'll be increased at 5500rpm, well out of the powerband that a geared 5.0 should be seeing. It'll also be costing hp and serious amounts of torque up until higher engine rpms.
Take a look at the primary size of the head's exhaust port, compare to a 1 5/8" header and you'll see what 86GT was saying about the lower velocity, and why they "tune" your exhaust flow. 1 1/2" unequal length headers are the best compremise for a stock engine, unless you go with long tubes.
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