Welllll so far, not much. I can boot DDR4-3333, DDR4-3466, and DDR4-3600. 3600 is not stable at all that I can tell.
The command rate setting is available in at least two different menus, and neither one seems to work. I'm stuck at 1T for now. That may be holding me back at higher clocks.
So many subtimings, so little time.
edit: I went back to the drawing board, reset to default, loaded up my volts, and forced XMP mode.
The first thing I noticed is that it tried setting my vDIMM back to 1.35v. I manually changed that to 1.4v.
The second thing I noticed is that despite the new RAM straps, it still set my DDR4-3733 to DDR4-3200 when engaging XMP mode. Near as I can tell, it set all the subtimings correctly (though a lot of them were on Auto, so maybe there's room for improvement there). DDR4-3733 had to be selected manually.
Using the timings/subtimings from the XMP settings, DDR4-3600 was mostly stable (y-cruncher crashed once, but everything else worked fine, and y-cruncher did complete a run of 500m). DDR4-3733 would boot and run SuperPi32m but y-cruncher brought down the machine with an Unexpected Kernel Trap or something silly like that. Didn't try anything else (it crashed). I am only using SoC voltage of 1.15v with LLC Level 3 (and vcore of 1.35v + LLC Level 1) so maybe I need to up that as well.
Still stuck on 1T.