Just got my batch 2 Mini yesterday. Ran it at 650 Mhz all night and had no HW errors and minimal pool rejects (using nanopool). Getting around 14.8Ksol/s. What's odd is I changed the cgminer.conf to set the speed to something higher but anything higher than 650 just gives me a really low hash rate, around 4.5Ksol/s, even 655 Mhz. No HW errors, just slow hashing. Anyone else experience this?
I noticed the Z9 mini seems to hash slowly unless you select speeds in specific increments. (usually 6 mhz) the only reason I know this is because after I experienced the same thing as you on my manually adjusted batch 2 mini, I looked at the OC options on my batch 1 mini and it looks like they usually let you select in increments of 6 or 7 mhz. I don't really know why but I'm guessing it's a memory timing thing. Try adjusting with the following exact increments until you get stability. 700, 693, 687, 681, 675, 668, 662, 650, etc.