Moved to LMM Tuner forum.
PPE, Edge, Diablosport, Quadzilla, Bullydog, etc all carry tuners for the LMM. They all have "mileage" programs available. Many of them have the ability to turn off the regen feature of the DPF, where you lose a lot of your fuel mileage.
You have to do your own research and find what tuner would work best for you.
As for violating warranty, lets just say that GM doesnt like tuners. That doesnt stop most people from using one, as you return the truck to stock before you go in for any dealer service.