As far as I can tell, using burrs from a Mazzer dramatically improves the grind quality (of course I'm only judging by pictures I've seen online).
Given the fact that I am using the La Pavoni Professional, that is; probably the most demanding domestic espresso machine ever in terms of grind quality, and given that I have been using one "
Modified KitchenAid Pro Line" next to it for a good deal of time now... Not mentioning that you are the one who's "judging by the pictures he's seen online"... I believe it's pretty clear who's the one who's "speculating". And by the way, I wasn't dressed up like you when I wrote that and I was
indeed wearing my pajamas

.
Yes, I will explain this to you, the slight difference in the outside diameter like I said won't enable the new Mazzer Mini stationary burr to sit in the groove of the Pro Line body. Probably you don't know that there is a circular groove in the grinding chamber of the Pro Line in which the stationary burr sits, which is by the way 3 whole millimeters deep. Forcing the burr to sit on the edges of that groove rather than to sink into it does make it look like around 2.75 millimeters thicker (because of the burr chamfer)... This is how the D-shaft is suddently forced to function in the far loose region, risking unplugging from the motor like I said. However, it may not be as clear why the grinding seems to be slower and the retention is so much exaggerated in the new configuration, which is briefly touched on below.
The size of the grinding chamber is increased significantly due to the improper seating of the new Mazzer Mini stationary burr, and because of the centering cavity in the bottom of the burr as well, there is suddently a plently of space for the grinded coffee to get into. The result of this is that the first whole double-shot deal of beans will get
swallowed by the machine

, and only after that, the grinding will continue, but will stay
significantly slower than before, and the vital question is why.
The inside diamter of the Mazzer Mini burr is considerably larger than that of the original burr, that is, 33mm compared to maybe 28 millimeters. The inside diamter of the original burr is DESIGNED to crush the beans by the auger upon entering into between the two burrs, this is how the static issues in the grinder are minimized as well as how the grinding speed is so much increased regardless of the sober 400 RPM geared motor. The new inside diameter makes the crushing less frequent, and increases the probability that whole beans will make it into between the two burrs, which is exactly why the grinding speed becomes much slower, that is, this isn't how the KitchenAid Pro Line was designed to perform. The burrs in the Pro Line like I said "fine tune" the already smashed beans, which also gives some insight into their one or two-phase pattern (I am talking about the burrs). I hope that these few explanations give you an idea about what I meant by going against the grinding chamber design calculations. They are simply the naive geometerical relations that make the whole grinding thing possible.
One more thing - Mazzer Mini replacement burrs are expensive compared to what? I checked yesterday and new burrs run about $40. To me that's a small price to pay a dramatic improvement in grind quality, but honestly I wouldn't bother unless I was intending to grind for espresso.
Expensive compared to stock cast burrs that you can make in-house in large quantities, rather than needing to search for some Italian manufacturer to source them. Quality machined burrs aren't a commodity, otherwise there would've been a lot of Zassenhauses (companies that make such fine hand grinders in which the grinding mechanism is mostly everything, which people request from all over the world), and like I said, the improvement is not like what you think upon incorporating the Mazzer Mini burrs in the Pro Line. This is (again) due to the poor mounting of the burrs in the vertical arrangement which prevents having a grind that is as even as the Mini's, plus the fact that the auger crushing still happens with the Mini burrs, though less frequently.
According to my personal experience, the bitterness
persists.
Best Regards,