update:
Picked up the truck yesterday afternoon. Turns out I had to pay 51 bucks and change (after tax) to have them re-calibrate the cpu to take into account the new tires.
Prior to the change, my speed was off by about 2.5 MPH at 60MPH (calculated by GPS). After the adjustment, I am still of by at least 1.5MPH or more. SO, in my opinion, his is very little improvement for my 50 bucks. Had I known that this adjustment would only bring my spedo 1MPH closer to being accurate, I probably wouldn't have spent the money.
For future reference here's the tech write-up indicating what the lower limit of the module is in our '08 vehicles:
26210 TIRE SIZE REPROGRAMMING PROGRAMMED TIRE RPM TO 596, THE LOWEST ALLOWABLE BY MODULE
For reference, my 35's say that the revs per mile is 593. Close, but no cigar
Jay