I have never tried the "Slingshot" method because I always felt that my support hand has enough to do with the next Mag and re-gripping. It seems to me that flipping the slide lock with your thumb is simpler and quicker TBS if you haven't tipped over the Perp with your first mag you probably are a bad shot, so the split second difference if there is one won't mean didley. As for Lefty's, I can see no other choice. I say whatever is comfortable for you is what you should do. Not what some trainer or coach tries to teach you.
