727 Transmission Efficiency...

Getting back to the original post, it's not unheard of to experience 10% slippage in a vintage stock converter. This is precisely the reason "lock-up" converters were invented, to improve driveline efficiency, and lower transmission fluid temps for better reliability. I believe I read somewhere that the first trans with lock-up converter was the Ultramatic built by Packard in 1949. Chrysler incorporated lock up converters in many vehicles in 1978 due to the fuel crisis. (Interestingly, the Chrysler units used the same principle as the Packard.)
The slippage is not (or at least should not be) inside the transmission, it's in the fluid coupler that connects the crankshaft to the 727's input shaft.