Tom,
Incorrect. There is no assumption that you can descend below the MDA on GA.
50 feet is a figure used in the criteria as a baseline, it is up to the crew to decide if the aircraft can meet the requirements for the real-time conditions.
Momentary descent is how far the aircraft descends down the flightpath, and is based on several factors.
Crew decides to go around.
Crew initiates go around
aircraft responds to go around, and
aircraft begins to climb.
The criteria is based on this timeframe taking 7 seconds, with pilot reaction time to initiate of 2 seconds.
If the engines are at approach idle, this adventure may take longer, or many other real-time factors.
With a chart showing an MDA or even a DA, the above must be understood, as there are many aircraft that will loose more than 50 feet in this timeframe.