Performance difference between manifolds and headers?

[68CST327]

In a few weeks I think I'll be ready to head to the muffler shop and replace my broken old single muffler and rusted leaky tubing. I'll be getting a dual pipe setup, with an H-pipe, and probably Flowmaster 40's or maybe Dynomax's, but I'm really debating whether I should just bite the bullet and have a nice set of headers installed at the same time or not. Right now I'm running stock ram's horns.

I've also been intrigued by the Corvette 2.5" manifolds. How much power do they make over stock ram's horns? How are they comparable to headers? From what I've read, headers sound like a pain in the @$$, yet they produce dramatically more power (debatable I'm sure).

Desktop Dyno 2000 claims an increase of over 50 horsepower on a 327 when using headers as opposed to manifolds. Is this accurate?