I've only messed with the extremes.
For me, it's in the feel, so no data there.
I have Stiff flex in my drives and I can swing hard, or (better yet) with what feels like my normal tempo when I want to be smooth (that doesn't mean slow, in fact, the smoother my swing feels, the faster my head speed - Trackman says so). I've tried X in the store and that 'feels' good or about the same to me....
When I hit a softer flex club (like my wife's) I am unable to swing my 'normal' swing, I'm forced to change up my pace and release and then typically get pretty decent hits anyway. I think I lose a little bit, but I'm not sure, my variability in my swing is likely a much larger contributor. I think if I tried my 'normal' tempo with her club the ball would just spray away.
However, I do feel the club reaction to my swing more obviously with her club, (I feel it load up and can feel the rebound.....or at least I think I can...)
With the S and X clubs, that 'feedback' is usually only there on really smooth/clean swings.....
So I use what feels the most natural to me (S and X) and that, hopefully, will play back in my level of confidence when I pick a club.
s...your brains will literally explode...