[Jeff Lee]

Back to the original question, stainless does offer a HP improvement. The pulses react differently to stainless than steel tubing. I can't say that from experience as I've never had the pleasure to own such a set. That's what my engine builder says. He also says some internal coatings (not ceramic based) are a good alternative. He DOES have the before / after to back up the stainless or coated headers vs. regular steel.
Maybe our engineer SSDiv6 can chime in...