This could be part of my problem with the 7870LE from Power color. It comes with stock memory of 1500mhz but I bet the latency is super high. It runs best with my memory set to the lowest possible amount of 750 lol.
That knowledgeable guy in the thread identified how the video card will use a set of different memory latencies, by looking at your memory speed, and then looking up up a table to see which set of latencies to apply.
Apparently, the table is broken up by sections of memory speeds. So from like 13XX to 1500 it would use one memory latency setting, but then from 1501-XXXX if would use another set.
Therefore, if you changed your memory speed from 1500 to 1501, that would trigger a new set of latencies be used, as looked up from that table.
What you could do, is try keeping all your settings the same, then change the memory speed just a little, to trigger the use of a different latency. We know that changing the memory speed by 1 MHz will have almost no effect due to the speed difference, but if you see a big change in hashrate, you know that your card is now using a different memory latency table entry.
Apparently this guy is so skilled/knowledgeable, he can re-do the video bios by completely re-writing the entire table to fix every one of the latency entries, so it doesn't even matter what memory speed you use, because every entry in the table will be optimized, so it's no longer a crap shoot trying to find the "sweet spot" in the latency table.
Pretty much a huge breakthrough, that solves the "mystery" aspect of why hashrates seemed to be unpredictable. It's just because different vendors were sloppy or lazy at setting up the table of speeds that should be used, and XFX was notorious apparently for this. However, some guy seemed to have a huge bump after he got the fixed BIOS from that guy that corrected latency setting table.
This is my understanding after reading the thread earlier this morning, maybe things have changed since then. I'd really like to know more about how to go in and do the changes myself to fix/optimize the video memory latency settings that are used at various memory frequencies...