I see that your sig says it is an 05. How late in the model year was it made? I know the 06 Silverado and Tahoe and the gmc counterparts and 07 classic Silverados, had a different wiring pattern and programming for the TAC module and TB. So if yours was late enough in the model year it is potentially possible that it got the same treatment. Symptoms of the wrong TB are reduced power warning once tps passes 50%, they seem to mask the issue until that point.
Or you have the 05 and earlier programming like it should have, and the TB is for a later model.
P.S. the plugs are the same, but pinned differently.