+1 on sunlit's comments.
Rear disc conversion is only for looks. If you want to improve stopping power start with high performance pads and if you want more, research the hydroboost conversion.
Your Tahoe utilizes a "vacuum" system to power assist the brakes. Heavy Duty (3/4 ton & 1 ton) models chevy made came with hydraboost instead of vacuum. Hydraboost systems tap into your powersteering pump.
From what I've read, hydraboost setups are the best bang for the buck and are a significant improvement. However, it takes time and research to do this the right way...b/c errors with the install can cause the brakes to fail.
For new AC Delco parts, you're looking @ $700+. Some guys will just take used setups off of heavy duty trucks and use that.