I have never personally run them, but a guy I knew put them on his dually. He said that it definately responded better and he clames he picked up a little mileage also. The biggest gain for him was the drop in egt's. He was able to turn the pump up a little more than usual and still not have the exhaust temps get out of hand.
I have always wanted to try making a set of headers for the turbo IDI's. I think it would help reduce if not eliminate turbo lag as well as drop egt's. Maybe someday when I am rich and bored I will get around to tryin it.