maybe a bandwidth issue. I would try running 1x risers on all of them
also check gpu usage in afterburner and make sure it's constant and not throttling up and down.
I believe you are right. I tried taking out one of the 16x slot cards, and the speed of the other 16x slot card rose to match the 1x riser card. Something about running two cards in the 16x slots is dropping my hash rates by about 10%. I'll order another riser today and give that a shot.
You're comparing the PowerColor to the XFX cards. They're not apples to apples. Different brands run at different rates, have different memories, different BIOS's, etc.
Notice that I swapped around the cards and still get the same results. Doesn't seem to depend on the make of the card.
Blank the 16x 8x and 4x presence detect pins with tape on the cards and force them to 1x on the 16x and see if you get more hash that way.
This is an interesting suggestion... do you mean I should mask the pins on the card that detect 16x, 8x, and 4x presence? I'll google it some more.