In all honesty, I love the custom tuning with the SCT from Rudy's. It offers much smoother power delivery and seat of the pants much more power.
With the H&S tuning, I would notice that the tail pipe would clean up at full boost no problem. With the SCT tuning I still have smoke not just a haze. The added power was definitely noticeable by me and my passengers even. Again this was canned tuning and not MCC. Id actually like to try MCC tuning if I had the opportunity.
As far as mileage goes, I couldn't believe the claims of better mileage until i ran it on a trip. The girlfriend and I jumped in the truck and made a 110 mile trip for a concert. I ran 80mph the whole way and didn't slow down until we got to the town we were headed to. At 80 mph with my 8200# truck on 35x12.50's (this is what i weighed when I got on the scales to sled pull this last weekend) I got an average of 22.1 mpg on the display and hand calculated to 22.07 mpg. I then left there and drove another 120 miles without filling up. I used only 1/4 tank of fuel total and got 21.4 mpg hand calculated.
All in all i feel like it was a great purchase. I really liked the H&S tuning also. But I ran a 6.0 before this 6.7 and the custom tuning vs canned tuning was leaps and bounds ahead of the canned tunes. The same goes for this truck. If I had a 6.4 I'm pretty certain I'd be running H&S. But I do love this SCT for my 6.7.
I hope this helps.