EDIT: I am using Asus R9 280x DC2 TOP version 1, I am getting around 780kh/s for each card.
May I ask your cgminer configuration, please?
+1
780kh/s seems very high
780 surely sounds kinda high. Could you share your configuration please and also show us maybe a screenshot of your miner? Not trying to bash, just wanted to see since I was planning on buying a few of those myself, but am still looking around at others. This would give me a 100% confirmation to choose those if one can still find them.