The pump may have that output but you can be sure the various piping and fluid passages are gonna kill that rate horribly. Look at the video, that is NOWHERE near a 50gpm rate. 50gpm would be like a wide open water hose.
While I have simply disconnected cooler lines to do one, I have not seen a clearcut indicator on one where the fluid suddenly turns color, the fluid is heavily mixed in the converter and there is no clearcut time to stop. Refilling while pumping out probably the best way, 10 quarts in and 10 out say.
I pull the pan and put drain plug in it, then either a 2X or 3X change to get it all. I even saved the output from a 3X operation to use it in a 2X on another car to save fluid costs. I also save 3X output to use for like chainsaw bar lube and the like.
For sure if you can find gasket it's far better than gluing up the pan........