Jim's right for the most part. The 15-20 HP will depend on the motor's origin. There are some combinations of cam and heads that would suffer greatly from factory manifolds. These would ONLY perform well with headers, and the difference would be huge. However, Jim's reference is directed toward factory combinations, such as your stock set up.
R.A.R.E. (Ram Air Reproduction Enterprises) reproduction long-branch manifolds are an improvement over factory long-branch manifolds, which are an improvement over factory log-style manifolds.
