I've been mining sph sgminer on win8 for a few hours now with 1x R9 290 reference model with no such drop in hash & they are not watercooled. Rock steady at 2.41MHs. I'm happy to help if you'd like to know anything about config etc.
My only issue is that the expected Darkcoin per day is abysmal :/ Approx 1.1coins..hardly seems worth it
Same for Hiro:

Gigabyte R9 290 WF3
12 hours mining, no drop in hashrate.
And the profits are acceptable (0.003btc in 12 hours, so 0.006btc in 24 hours, but at a much lower power consumption, wear and tear than the 0.01 Adaptive Nscrypt Spaincoin/Rotocoin).
The best scenario would be that yo see no drop in Windows 8.1 and a drop in BAMT 1.6