Do you have verified horsepower numbers from a dyno test? 50 sounds awfully high, more like an urban myth.
Consider the first Ford-O-Matic, in 1951. It was cast iron and almost as big as a C6, and not nearly as efficient, I'm sure. So we can assume it required much the same horsepower as a C6. A '51 flathead was rated at 110 horsepower but likely made less. Thus, 50 horsepower would be half the total output! In reality, they performed about the same as a manual transmission; a little slower, but not 50 HP slower!
Many 40 HP Model A's have C4's installed. If those trannies drained off 25 HP, that only leaves 15 to move the car! Does that even make sense?! I don't think so!