How much has mizuno technology changed in their forged irons since 1990?

Not much... but it isn't just Mizuno. Iron head designers only have so much room to work with. This is because the consumer doesn't like the look of "weird", even though it could actually perform as advertised (think of the old Browning 440's or the Nike SuMo irons). They can only make the blade length so long, the topline and sole so thick, or the offset so much (all methods of helping correct offcenter contacts) because that also falls under "weird". They're left with trying to do things within the same basic shape... so what happens? Not much. They have cut-muscles, "pure" muscles, cavity backs... but each only offer so much, with very little variance. The CB will have slightly more forgiveness and possibly a slightly higher launch angle (with all other specs being the same)... but the key word is "slightly".