Well, I'll take a shot though there are many here that are much more knowledgeable.
First, I just made the same upgrade, a Rocky doser to a M4 doser. I upgraded mostly for the hopper size, dose adjustability (though it doesn't look like I will be using that feature, I just dose to fill) and grind speed (I often have to make 4-10 drinks). I did not upgrade for grind performance as I am making 90% milk drinks, the subtleties of grind are lost in a latte or other. So I guess the question may be is what are you drinking mostly? If it were me and I was drinking mostly milk drinks the Rocky would be more than sufficient.
As for skipping the MACAP and going straight to a Mazzer, this implies the MACAP is a downgrade. I have to say this is not the case. Mazzer has a bigger name because they came to the market first, but as can be seen
here the MACAP holds its own against the Mazzer and IMO has some distinct advantages (timerless on/off, reports of leaving less grounds in the doser), plus it's a bit cheaper.
My only complaint about the MACAP (and it may apply to the Mazzer as well) is that the doser lever is quite a bit louder than the Rocky's. The grind is a bit quieter though.
Fr. John