Quote:

... Most guys out think themselves and don't realize they need back pressure.


Sorry, but that's crap. Back pressure is BAD; you want as little back pressure as possible in a performance application.

What you have to take into account is most engines (especially in near-stock configuration) aren't tuned for a no/low-backpressure exhaust system. That's why some people mistakenly think that some backpressure is good... because they haven't made the rest of the tuning adjustments to handle an improved exhaust system.

Re: the original question... yes, a good set of headers are definitely worth power over a set of OEM HP exhaust manifolds. As others pointed out, how much depends on a lot of factors. However, on something like a mild-to-moderate 440 street engine, I'd say it's a safe bet that switching from HP iron manifolds to a good set of properly-sized headers for the application would be worth 20-40 HP and a comparable amount of torque.