ok, so i had two dual-GPU machines crunching MW@H recently. one had dual Sapphire 7970s (not the same exact models though), and the other had dual Gigabyte 7970's (matching GPUs). the fact that the Sapphire 7970s didn't have matching model #'s is irrelevant though b/c they both had the same amount of memory (3GB) and were clocked the same. so, without further ado:
Sapphire 7970, core @ 950MHz (factory OCed), VRAM @ 800MHz (underclocked), BOINC v7.2.28, Catalyst 13.9, Win7 x64: ~67s
Gigabyte 7970, core @ 1000MHz (factory OCed), VRAM @ 850MHz (underclocked), BOINC v7.2.28, Catalyst 13.9, Win7 x64: ~69s
...now i know it doesn't make any sense that the average run time for the lower clocked Sapphire 7970 is slightly better than the average run time of the higher clocked Gigabyte 7970, particularly since the supporting hardware is all the same (both use the same CPU, mobo, type and amount of RAM, etc.)...but i suspect that the small sample size of 5 tasks might be the culprit. i used to have a back log of hundreds of valid tasks on each host, but those back logs have dwindled to ~60 valid tasks each since having pulled both hosts off the project 4 days ago...so unfortunately i can't simply add more tasks to the average without getting back on the project again for a few days...and i can't do that either b/c those two hosts have since been completely taken apart. at any rate, i suppose you should post the better of the two times i reported...