Your tahoe PCM shouldn't be any different than other ones. I'd send them an email and see what's special about yours. I just looked on there actually, for the 07-08 Tahoe, I didn't see anything special, other than 07+ is recommended to use at least 89 Octane due to higher compression.
As far as the mileage is concerned, I'm sure they're just covering their butts for any damage caused by tuning and parts failing due to added power........... like the trans lol. I can't blame them, I'm sure someone's tried it before.
Edit: Sorry to sidetrack your thread Mountie!!