Had a go on this as part of my fitting recently. Very impressive and it really does feel like a normal club. Gave me DGS300, PX5.5 and KBS Stiff.
Going with the S300 as it gave better flight.
Bit of both. Marc at Oxford Acadamy looked at the flight from the shafts suggested by the Optimizer - saved a bit of time choosing between the plethora of shafts available.
All 3 swings were within a whisker of each other readings-wise. Once we knew we needed stiff shafts we concentrated on the 3 mentioned and chose the best of them. I definitely needed stiff rather than PX5.0 that I have currently - it makes a huge difference.
Mizuno makes several weights of standard head, mainly to allow for adding/subtracting length and maintaining swingweight. I would imagine that most fitting places do not have those though, and use standard head weight.
What I was getting at was - once you have found the right shaft for you at (say) a Mizuno c/f session based on Y or Z head, would that shaft still be the best one for you if you decided to go for another manufacturer's club head. i.e. find the right shaft, find a club head that you like, and combine the two.
Maybe but different heads having different designs and weighting would affect trajectory to an extent.
Having a DGS300 in a MP68 doesn't necessarily mean you need a S300 in a Cally X 22 - the two heads are designed to do 2 different things.