OK Birdman, here's the FR on the MP58s as requested.
I oni have the 4 & 5i for the MP58s in the bag rite now and I dun really hit it as much as I wud like. Their impact somehow just feels that little bit different from the 68s. I wud say its more clicky at impact and not as soft as the blades. I wud expect that cos the rear insert is metal and not some polymer or rubber damping device. Its also to be expected from a CB compared to an MB.
That said, because its a cavity back, the trajectory of my 5i is almost the same height as my 6i. I suspect the titanium insert and weight near the sole has something to do with that.
Comparing the 58s with the Titleist 710 AP2s (not the 710 CBs although I've hit them quite abit on the trackman too but comparison will be done on irons with rear inserts)
and trying to be as unbiased as I can, I will have to say that the AP2s will 'feel' softer not because of the forging or type of carbon steel but because of the elastomer insert at the back of the club.
The sole of the AP2 is also noticeably thicker too. Likewise for the topline. The 58s' sole width and offset hardly differs by much from the 68s making the combo set look really uniform at address or when viewed from the sole. There is also a little more bounce and camber on the Mizzy's sole and its complemented by a blunted leading edge so it really favours those who tend to dig on their iron shots.
Conclusion: The 58s looks really good at address with its noticeably less offset and thinner topline and sole than the AP2s and this will be important for players who are finicky and prefer to look down at an iron with those aspects. However I will have to say the AP2s will be a tad more forgiving and might have a fraction bigger sweetspot than the Mizunos. That rear insert really takes away quite abit of vibes on off centre hits.