Who is your local shop ? One of the "name" players in the Duramax field, or just a local shop ?
To flat out make promises is salesman talk as there are no guarantees with Duramaxes.
Do yourself a favor and join here:
http://www.dieselplace.com/forum/
and here:
http://www.duramaxforum.com/forum/index.php
Get into the subforum for your generation engine and do some asking and talking before you spend your money. There are lots of folks that have done what you are planning on doing and can offer feedback on which brand components actually deliver said promises/results.
You know, on one tank of diesel I got 2.5 to 3 extra MPG simply by adding some cetane enhancer and watching how I drive. That was for a $10.00 investment, but in all honesty, it also happened to be a good batch of fuel also.
I have added components to make my truck better, in my case to cure the overheat issue the LLY's have. A tuner, a better intercooler, bigger hot and cold side pipes, the 06 air box, the 06 turbo mouth piece.
The truck runs better, doesn't get hot like it used to and might have picked up 1 mpg, might have.
It seems the MPG figures our trucks get vary wildly truck to truck. It just seems to be luck of the draw.
My point is, doing a little research on the forum will not cost you anything but a little bit of time.