The things that make Rifles 'da coolist!' is that they come frequency sorted when you buy them as a set - meaning that each shaft will play the same as the shaft that preceeded it - a real boon for clubmakers.
The Dynamic Golds have around the lowest ball launch of all iron shafts.
Rifles get the ball up higher, but not as much as say, mercury savages.
Now, this said, the 6.5 is similar to the DG X100's.
For the S300, you'd probably want a rifle 6.0 or 5.5. You'll be able to hit the 6.5's, but your ball flight will either be the same or lower than your S300's,
shaft for shaft. The other thing to keep in mind is the heads. The
clubs that come with the Rifles may be a higher launch head than your 32's. I don't know much about
Mizuno sticks, so I don't know what type of head or how much offset the heads have.