There was no direct discussion. This is a group opinion.
I know for a fact GM retooled the Vortec 5.3 in '03. This included raising the piston rings to a higher point on the piston. GM made changes to the Fuel MAP when Mass Air-Flow is ignored and only the Manifold Absolute Pressure Sensor is used when Wide Open Throttle. GM also changed to a slightly higher-flowing Catalyst System. In '04 GM went to a returnless Fuel System, but no one I know can determine why this would be a power increase. We do know the '03 and '04 lay down similar Rear Wheel HP numbers when the same gears and overall tire size is used.
Different info gathered from MANY different sites.