A shaft is as more about feel and result than it is swing speed. Hitting several different shafts should give you a better idea of the "feel" you prefer in your own swing. Balls, grips, gloves, shoes and the lot are chosen more from feel than any other factor... why should your shafts be any different? If you have time or motivation to try a bunch of different shafts on a launch monitor then you will likely be rewarded with better fitting equipment for your swing and thus better results.
FYI from the Golfsmith website if we are discussing speed and flex.
Shaft Flex Extra-Stiff
Swing Speed (mph) 105 +
Carry Distance (yards) 260 +
Club from 150 yards 8 or 9-iron
Shaft Flex Stiff
Swing Speed (mph) 90-105
Carry Distance (yards) 260 +
Club from 150 yards 6 or 7-iron
Shaft Flex Regular
Swing Speed (mph) 80-95
Carry Distance (yards) 210-240
Club from 150 yards 5 or 6-iron
Shaft Flex Flexible (A-Flex)
Swing Speed (mph) 70-85
Carry Distance (yards) 180-210
Club from 150 yards 4-iron
Shaft Flex Ladies
Swing Speed (mph) Less than 70
Carry Distance (yards) Less than 180
Club from 150 yards 3-iron/lofted wood