I have played a little more with dressing the Matrix stones on flat stones. What I used were "granite" tiles I machined flat since the only bench stones I have are for deburring flat surfaces in my machine shop and are NOT used for anything else. They worked ok, developed some mud, and cleaned up the Matrix stones ok, just ok. The ends of the Matrix stones are not getting cleaned up very well but otherwise look good. Still, I think stones are much better than sandpaper. I am back to "loose abrasive on a flat plate is by far the best way to dress Matrix stones", nothing else is even close. Here is a photo of the Matrix stones on the stones I used to dress them, please dis-regard the water spots. I spent a lot more time dressing on the stones than I would have if I had used loose abrasive.
I finally measured the Matrix stones that EP uses daily, something I am supposed to do every May but it just hasn't worked out. Unfortunately, I didn't write down what they started out at on my list, this info is engraved on the stones, but have a good idea of where they started so I will give my best estimate here and edit it once I get the info. They started using the 250 through 1100 grits about March of 2018 and the 80 early February of this year.
80 grit - lost .001" in 6 months, but they don't use it very much.
250 grit - lost around .003" in 17 months
650 grit - lost around .015" in 17 months
1100 grit - lost around .014" in 17 months
2300 grit - lost around .004"-.006" in the last 15 months
4000 grit - lost around .002" in the last 15 months
Cody did say about a month ago that he figured they were going to show a lot more wear as he has taken to dressing them after every 5 knives or so as he likes how they work after a fresh dress. They keep their dressing plate next to the sink, which is next to the sharpening station so it's real easy for them to give the stones a quick dress. They sharpen around 50 knives a week and use the Matrix stone most often. With proper care and dressing, I would think the wear rate could be reduced by 80% or more. If you want the stone to cut faster just use the next coarser stone since freshly dressed stones leave deeper scratches anyway there isn't that much difference.
I really don't think using a diamond plate to dress the stones is a good idea. It may kind of work because your only really removing resin on the Matrix stones but it will be hell on your diamond plate. Even if it works well at best it will be 20% as good as loose abrasive on a flat plate, which is about where stones and sandpaper lie.
I have made Matrix stones with diamond down to 1 micron but they really only work down to 5 micron, or 4k grit, on steel. On ceramic knives, the 2.5 micron works really well. If you want to go finer then I suggest doing it with a strop, which is what I do.