I was recently hitting the cleveland 588 irons (because they look so great) to see how I would fair. I had a 6 iron with s300 shaft. I noticed that even on shots where I felt I struck the ball pretty pure, they went 15-20 yards shorter than my current irons. I normally hit a 6 iron 175 yards, but on well struck shots it was only going 155 yards. 160 yards at most.
I know these are more of a player's cavity, but they are still relatively forgiving. I can't imagine I was totally missing the sweet spot on the shots that felt pure and solid to me. I currently play ping i20s with the stock CFS shaft.