DavecUK I can elaborate more on why the SSP HU are better as a single doser and it comes to pure design.
They are better in catching the beans, See the pic below:
Those little funnels catch and funnel the beans to be grinded.
The Cast has some form of these funnels so most likely they will fit for an espresso grind but that is unknown at the moment and SSP recommends them for filter.
The MP don’t have them:
So you need to grind coarser to create pathways for the beans to flow in.
If you look at the mazzer burrs they all have them: