From Tom Wishon:
I hate to tell you this, but the reason you don’t see much difference in these shafts is because when it comes to iron shafts, there really is very little difference in terms of outright visible shot performance differences between shafts of the same flex. Shoot, with many golfers, they could use a full flex softer iron shaft and never notice any real difference.
The reason is because in terms of how much iron shafts do actually bend during the swing, it is not very much – half of how much shafts in woods bend during the swing. So with a hugely reduced amount of actual bending in the iron shafts, there just is so little difference in real performance.
Only players with very high speeds and very late releases who also have a very refined sense of feel, and who are also very repeatably consistent with their swings will typically see much in the way of actual shot shape, ball flight differences between iron shafts of the same flex measurement.
Now with shafts for woods, it is different because shafts for woods do bend on average twice as much during the swing as will iron shafts of the same letter flex measurement. Lots of players can notice differences in wood shafts. But not so much with the irons.
TOM