I have a 2003 Silverado K1500 with the very common LM7 5.3l engine. Modifications include cold air intake, Throttle body spacer, cat delete dual exhaust, 721 Cadillac camshaft, and electric fans.
My truck has 50000 km on the tune that Jason has installed. It has proven to be very reliable, harsh +40 Celsius summers to painful -40 Celsius winters.
The mileage has increased from the stock configuration. I can regularly expect 65km more per tank in the city, and 110km more per tank on the highway. I frequently use my truck for pulling either a 27 foot holiday trailer, or an enclosed car hauler filled to capacity.
I would recommend a tune from D&M tuning. It has worked out very well for my needs.