Do you know what shaft you had in the Mizuno's? Swing speed of 75 is in the senior flex range, but it is what it is. Mine swing speed for a 7-iron is about 83 and I thought originally, I was going to end up with a senior graphite shaft, but I ended up with a regular flex Elevate 95 gram steel shafts. However, the other day, I hit a few balls with a 120 gram stiff flex and each one went far, high and straight. I don't think I would ever switch off of my shafts and if I did, it might be to a 95 gram graphite shaft, for greater height, but those 120 grams felt good.
Generally, about 80% of golfers can hit the ball farther with a heavier shaft, but the downside is if the shaft is too heavy to consistently square the face up at impact. Some golfers (like 20%), with slower swing speeds, can generate more ball speed with a lighter shaft, because they are taking weight out of the shaft and thus making the club head feel a bit heavier. Every golfer is different.