IMO: I would say there is a very noticeable difference in acceleration and throttle response. Take if for what its worth.
I stood behind the car with the stock and the NHP exhaust WOT. Video doesn't do it justice in the difference of sound and the quantity of hot air being blasted in my face.
Also, I am in no way trying to promote NHP. I think any decent header and exhaust set up would see the same results.