Also, i am hesitant to actually have swing speed as the determinant for how stiff a flex is. That's kinda a general idea. But a X-stiff shaft can have a high, medium, and low trajectory depending on kick point and how stiff the shaft is at the tip of the club. In general a X-stiff shaff will lower the ball flight, but it can vary greatly between the different X-stiff shafts, even stiff shafts can have X-stiff properties. Also, x-stiff from one company isn't the same for another.
I am also thinking that tempo might play a part in this. Someone like Freddy Couples or Ernie Els might use a less stiff shaft because there swings are smoother, but you take a Tiger woods, who has a very aggressive motion, that might require a stiffer flex. To me, the initial transition between the back swing and downswing might be a good indicator for they of overall flex. A person like Freddie might load the golf shaft with a ton of energy but does so in a gradual fashion, while tiger loads it very fast at the top of the backswing and maintains it through impact. So in Tiger might need a golf shaft that responds to his swing characteristics not his swing speed. This is just theory, but i think it makes sense.
So the point, go to a demo day or trackman and try out as many golf shaft variations as possible and see which ones work the best.
I have played a V2 extra stiff shaft, then switched to a Stiff Diamana, i like the stiff flex better. But my driver swing speed would classify for X-stiff
As for Iron flex, i would check out online shaft fitters, they give you a general idea. Like Project X has a good one, and so does True Temper. I am a personal fan of frequency matching myself. I think it really helps even out the flexes through out the set and gives a more consistant feel and ball flight.