The OEM burrs are cheap and easy to replace. I've never seen any aftermarket upgrades for this grinder though.

I am using Macap M4 doser grinder. I found one guy modified his Macap M4 to use Mazzer mini 58mm burr when I tried to use better burrs once.
He had to tap holes on upper and lower carriers to put mazzer 58mm burr since those two burrs have different screw hole patterns. I wanted what he did but gave up. SSP has 58mm burr for Mazzer mini doser only.

I recently replaced the burrs in my 14 year old M4 with the OEM replacement ones. I had to buy them from an espresso parts store in Germany though because all the supply houses in the US were out of stock. My advice - if you can buy them just get them. Easy job to replace. There are replacement/cleaning videos for the M4 that show how to do it. And, the new burrs definitely were better than the 14 year old ones that had ground 1000+ lbs of beans and had degraded and dulled.

It is still a good, but not great grinder, and best for drip or pourover coffee which is what my wife uses it for (her Moccamaster). I was using with my Decent and it was OK and could pull good shots, but the Niche gave me a step-function improvement especially for dialing in shots and using lighter roasts. My personal opinion is that spending the $150-180 for ssp was not going to give me what I was looking for in improvement. http://www.espressotool.com/786402328/?idx=3 But, it could be a fun experiment.

BTW, I bought my M4 from Chriscoffee as well. Here is what I ordered in case you cannot get from CC. https://www.theespressoshop.co.uk/en/Ma ... -3682.aspx