I can answer this one pretty good.
I just got new irons this year. Previous to that I used ping eye2. This year I've hit more greens and have eliminated the big hook that would arise.
Better ballflight too. Some of that comes from the fact that the old irons were made to perform with old balls. Just like today's irons and woods perform better with newer tech balls.
This year I have not shot over par with them.
I did a test...took my eye2 irons and hit both a pro v1 and an old Titleist Professional. The pro v1 went high and spun back off the green. The professional actually went a couple yards further, lower flight and pretty much stayed within a foot of its pitchmark.
I think if u buy new irons every year...or 4 sets a year, u will not be able to see any improvement.