I agree mostly.
The loss of power on a manual drivetrain ranges from an absolute lowest of 13% to as much as 16% from what I have seen, but the loss on an automatic varies a lot more. I have seen factors ranging from an absolute low of 15% to a high of 25%.
The stock 03 and 04 5.3 L SSR powertrains, which come ONLY as automatics, appear to deliver a net of 235 to 250 hp at the rear wheels, which translates to 17% to 22% loss. I donot believe that the engines vary this much in output, but rather that individual dynos are not calibrated correctly and identically, and that individual trucks have cleaner or dirtier air filters for example.
However, there is another factor that affects things. General Motors appears to use a more conservative rating standard than most dyno shops use. Dyno shops using Dynojet dynos usually use STD horsepower, which corrects the measured horsepower to a 66 degree standard temperature, whereas GM uses a stadard that corrects to temperature in the mid 70s (I can't remember the exact temperature, as I read about this some time ago). The difference is larger than you might think: a full 5%.
So, when GM says "300 hp" GROSS at the crank for the 5.3L, a dyno shop would say 300 x 1.05 = 315 hp GROSS at the crank. Those same dyno shops measure SSRs (and Silverados using the same engine) at 235 to 250 hp net at the wheels, which implies an even larger loss factor.
When I had my Z06, I noted that the 405 GM gross rated Z06 engines delivered 335 to 360 hp at the rear wheels on most Dynojet dynos, with a MANUAL transmission (only way the Z06 came), for a loss of 11 to 17% against the 405 hp rating, BUT after allowing for the 5% difference in correction factor, that loss becomes 15% to 21%, even with a manual transmission!
Best advice: Use the SAME dyno operated by the same technician each time you dyno your car, and look for the DIFFERENCES between runs.
To show you how crazy this can get, if you look at my dyno curve for my very mildly modified 04 SSR published on this board recently, you would conclude that my new GROSS ratings for hp and torque can be anywhere from:
Horsepower: 331 to 354 hp gross
Torque: 363 to 386 ft lb gross