I found the following web-page useful.

http://alexanderhuzar.angelfire.com/files/ram_timings.htm

HBM2 is SDRAM, so I assume similar discussions about SDRAM timings are applicable here as well. You'll note that the yellow "timing" column lists and discusses many of the parameters we're altering here.

Edit: one of the interesting things I've gathered from these tests (apologies if it's obvious to the more technically inclined) is how "subjective" pure memory frequency is. I've been quick to hate V56 stock memory, and hynix stock memory, because it's "slower", at least in terms of pure frequency. But what you notice when you look into the guts of the ram timings, is that the V56 memory timings (both Samsung and Hynix) are much tighter to begin with, which helps explain why you get higher hashrates at lower memory Mhz frequencies.

Perhaps there is little point in flashing Vega bioses anymore, now that this tool is available. Tweak the timings you have, and the "effective" data read/write speed will likely converge for most of these gpus, at least on memory bound algorithms, anyways.