Dave S,
From what I have read, step headers will, in some cases be worth a few Hp - but not necessarily in a street driven car/engine. Stahl is not a big fan of them and this guy, the second listed, not sure of his credits, also does't feel that a step header is a real benefit - see #5. It makes sense to me.

I went to college with Jere Stahl - and he was developing interesting stuff w-a-a-a-y back then and was the class car geek.

http://www.stahlheaders.com/JpgFiles/issue3.pdf

http://www.x-pipe.com/Scavenge.html

Methinks(I know - archaic, but I'm an OF) that step headers need to be developed with a certain airflow - camshaft and head design and an RPM range that the engine will turn most of the time and that just welding up a set of pipes at arbitrary lengths without airflow/dyno tests wont do it and that Hp might suffer

If you do build them, please make sure that you keep the forum updated as I hope you can prove these guys wrong .