Strange PG AMD GPU Results

[FourOh]

I recently picked up an R9 280X, which I have put to use on Primegrid in place of an HD 7950 in a dual Xeon machine. In an AMD FM1 rig I have an HD 7970 also running Primegrid.

The 7970 is a reference card, clocked at 975/1375. The 280X is an MSI Gaming card clocked at 1020/1500. The 7950 is a Sapphire Vapor-X clocked at 950/1250.

I anticipated the 280X would outperform the 7970 by a small but noticeable margin, but in fact the 280X is 30%-40% slower on PPS Sieve and 10% slower on GFN World Record. In fact the 280X is barely outproducing the 7950 it replaced.

The two cards are on the same, most recent, drivers; both on PCIe 2.0 x16 slots; both showing 98-99% GPU usage when running 2 PPS units or 1 GFN-WR unit. I have 2 threads (one core) dedicated to GPU work on the 280X and one core on the 7970.

Any thoughts on why my 7970 is beating the crap out of my 280X?

Edit: I suppose in the interest of science I should swap the 280X and 7970 to see if the platform is the issue; then isolate variables from there. But, I was hoping someone could just tell me