[blucar]

The value of headers on a street engine is another myth that just wont go away.
Headers are intended to be ran as "an open exhaust, no mufflers". As a general rule headers do not start to do their work until the engine approaches 3,000 rpm's or higher.
Stock cast iron manifolds with 2" dual exhausts will provide all the performance that a street engine can muster.
Of course stock manifolds do not provide the bragging rights that headers do.