Just to throw a spanner in the works! I was fitted for a new set of irons 10 days ago at The Belfry. For weeks prior i was convinced i was going to get the MP-59's. Upon arriving i hit both the AP2's and the 59's. I hit both with varying shaft combinations but just felt that the AP2 'fitted' me better! In some ways i was disappointed as i really wanted the 59's due to the rave reviews and their superb looks but with the AP2's i was getting better carry distances and better dispersion figures. I also found the AP2 easier to hit which was suprising.
As the guys above have said, if you hit your current Mizuno's well i would be inclined to stick with them as there is no doubting they are an incredible set of irons.