I posted on this yesterday but the bullet formatting did not work. When I edited the post apparently I deleted it, or perhaps someone deleted it for me. I will try again …
Years ago, before the large springy drivers and strong lofts in irons, a robot study was done for a swing speed that was marginally between regular and stiff flexes. They studied a 5 iron distance of about 160 yards and a driver distance of about 240 yards, comparing stiff versus regular flex shafts.
The distance and dispersion increased going to an R-flex in the driver, but the 5 iron distance was actually slightly less in the 5-iron. Still negligible, in my opinion, but it seemed obvious that the impact of the dynamic loft increase with flex exceeded the extra kick associated with flex at that loft angle. With the driver the dynamic loft was not enough to counter the extra shaft kick, or perhaps that dynamic loft helped produce more carry. I don’t remember anything on the actual lofts being studied.
I thought at the time that the difference in dispersion with both clubs was not enough to be significant for all but players at the highest level. The conclusion, for me, was that anyone near this distance range would not be hurt playing stiff shafts in the irons, but should consider the softer flex in the driver/metals. (Today, if your 6-iron distance is 160 you are probably right in the range of this study.)
There are, of course, many variables in this question and I think it best to test specific equipment for your particular swing in a controlled setting, such as on an indoor simulator where you can see the numbers. (I’ve hit stock “stiff” shafts that felt like a senior flex. I’ve hit stiff shafts that felt like an X-flex.) Generally you may find that the shaft flex question comes down to your tempo. Quick transitions at the top of the back swing and/or swings with exceptional shaft-loading typically demand a firmer flex for control.
I’d like to share a study on this that I read about many years ago. Perhaps some may remember it. I can’t remember the source, probably one of the golf magazines, but it made sense. There may be more recent studies and I’d be interested if others can provide a source/link.
I think the broad answer to the question is that “it depends.” Generally a softer flex adds distance potential but reduces control. I remember reading that “one should use the softest shaft that one can control.” It was a pretty subjective conclusion.
The study compared a five iron distance (probably with a loft closer to the older standards) and a driver distance (with old technology) using a robot. The tested swing speed produced a driver distance of about 240 yards and a 5 iron distance of about 160 yards using stiff versus regular shafts. I thought this was, at the time, close to the regular/stiff decision point. Results (from my memory):
Dispersion was greater with the regular shaft but (in my opinion) negligible. I think it was perhaps less than a yard for the 5 iron. As I recall, dispersion for the driver was also greater but not enough to be significant for anyone that would even be considering regular shafts (i.e., non-professionals).
Driver distance was a few yards greater with the regular shaft.
Five iron distance was slightly less with the regular shaft, but still very small, not something one would notice.
Why would the 5 iron produce less distance with a regular shaft? My guess is that the shaft flex added enough dynamic loft as to reduce the overall distance that one would get from the extra shaft kick.
This convinced me that, for many average players, stiff shafts for the irons will not hurt. But … for the driver, and probably fairway metals, one should consider erring on the softer side.
A lot has to do with how a player transitions from back swing to down swing. Faster transition swings typically suggest erring on the stiffer side.
In the end, in my opinion, it’s a good thing to compare in a controlled setting for your swing, such as on a simulator. There are a lot of factors that make answering the question specific to the player and equipment. Shaft flex, for one, varies significantly in lots of ways. “Stiff” versus “regular” actually says very little these days. I’ve seen stock stiff shafts in drivers that felt like a senior flex.