Sapphire Radeon R9 280X Dual-X been better than my gigabyte
saphire 750khs 1.025v much cooler than Gigabyte R9 280x Rev2
gigabyte cant get voltage below 1.07, hash only 735khs
and my sapphire finds somehow 25% more shares than GIgabyte
Is your Sapphire Dual X stable on that 1.025 voltage with 750kh? I doubt it is. If you flash Gigabyte R9 280x Rev 2 it can get below voltage 1.07 but its not stable. (or i havent find right settings to make it so)