The bottom line is that a 7990 is still the best performer. It even bests the new 10XX's.
Here's another teaser. My other dual card rig is running three tasks per GPU in almost exactly 31 min per task. I'm running twelve with two cards, thus six per card. therefore if you divide 31 min by six (I'm completing six every 31 min) you get 5:12 per task average time.
I'm lovin' it!