Headers vs 340 manifolds
I think that this is 100% BS
I would bet that your customers are reacting to sound and such (I'm glad they are happy!).
With the exhaust system hooked up, I don't think the difference would be noticeable in terms of acceleration. I think 15hp would be optimistic and on a stock 340 with around 300hp it's a max 5% boost. Take the exhaust down and you would either have to re-tune to take advantage (and then what happens when the exhaust is put back up?), or the engine was poorly tuned to start with and the open headers were reacting to a chance improvement in tuning.
Then if the car is mainly a street car you have to live with all the issues of a crowded engine compartment, smashed pipes from speed bumps, etc. After years of 5 different brands of header, I scrapped headers on my street cars and went back to well chosen manifolds and a well designed exhaust system. I never looked back...