I'm picking up a set of Mizuno MP62 irons but which shaft is right for me?

I've never had my swing speed measured with an iron but with a driver my swing speed is 88 mph with a moderate tempo. So my swing speed with an iron may be a bit quicker. I currently have a set of King Cobra II Oversize irons with R-flex, though it seems the shafts are also a little larger. I can get a set of MP62 with DG S300, would that be too stiff or should I really be looking for R300?