I don't know but Ive been told that stiffer the better. What would you want your #1 power accumulator to hit the ball with? A soft noodle or a baseball bat? I use stiff steel shaft in my short irons that I "Hit" and a softer flex from mid to long irons. I think a stiff top for #3 works well to. No answer for swing weight.
I don't "Hit" mch these days, I'm sure an answer from the Hitters will be posted soon.
Regardless of swinging or hitting although there could be an advantage to a stiffer shaft weaker whatever for either or but for the most part the only thing that matters no matter how u r moving the club ate the launch angle swing speed and spin being properly matches to the shaft when I hit I take a large pivot and stress the crap out of it I use x100