I had a Smarty set on level 3 on 2006 6 speed. One trip towing my 8000 lb travel trailer and the clutch slipped. The truck has 27000 miles on it.
It slipped in 5th and 6th gear pulling up a not very steep hill. I stepped down on the throttle and it picked up about 200-RPM's and didn't go any faster. I have owned the truck since new and never have slipped the clutch. I was very happy with the Smarty, and saw about a 1. 5 mpg improvement in fuel mileage both towing and empty. The extra 50hp was perfect for towing, bottom line is it did slip the clutch.
I spoke with Rip at Source Automotive and he said" I guess you got one of the crappy factory clutches. " I set the Smarty back to stock and hope I'll be able to nurse the clutch for a few more miles. Their will be a new South Bend clutch installed soon and it will be back to the Smarty.
Understand that I was very happy with the Smarty. Just be aware that because the Smarty instructions say you can run it up to level 5 and spare the stock clutch, it may not be the case.
Jeff