I would try to see if you can video the tachometer while taking off from a stop and then show that to the dealership. Also, did you get to go along on the test drive so you could "coach" them on how to make it happen? I don't trust anyone anymore, so I wouldn't be too sure if it happened or not without actually being there to observe.
You might also call GM corporate headquarters and let them know the names of the dealers. You would be surprised how many times dealers fix problems they said were nonexistent when they get a call from GM.
2011 GMC 2500HD Duramax Z-71 CCSB SLE
S&B Dry Cold Air Intake
Ranch Hand Front and Rear Replacement Bumpers (Ridgid Dually Lights on Rear (2)), NFAB 6-Step Nerf Bars, RKI Tool Box, Custom Headache Rack w/ Ridgid Dually Lights (4)
Limo Tint All Around w/ Limo Brow
EDGE CTS Insight, Rancho Steering Stabilizer, Backup Camera
3/4" Lift via Torsion Bars (it was all I could get with stock keys), 285/65/18 Nitto Trail Grapplers
GoLight Stryker HID
XRT Pro on Tow with 4" Flo Pro Exhaust