Half way through last season I changed irons on the advice of some very good golfer I play with. I went from 714 AP2 with stiff shafts to 718 AP1 with regular shafts and my handicap started to climb.....I was an 11 handicap and currently at a 16, but now progressing back towards 11.
This season after 5 rounds I went for a swing evaluation at Golftown and found that my ball striking was better with AP2's compared to my AP1's. My approximate swing speed currently is 77mph. The fitter indicated regular shafts might be a better fit, but my ball striking was better with stiff shafts.
Expensive lesson for me....