About 6 weeks ago I purchased a R9 280x. I spent hours trying to tweak it, and the most I could get out of it was around 550 kh/s. I couldn't remember which make it was exactly, and was too lazy to pull the computer apart to find out. So anyways, went and bought 3x Sapphire Dual-X R9-280x's yesterday. While installing them I noticed that they were the exact same as the card I already had in. When I got it up and running and started mining I was instantly getting around 725 kh/s on two card, the third is only getting 640 kh/s, and the old one is still at 550 kh/s. I'm guessing my first card was defective, and that one of my new ones might be as well. Has anybody else experienced this? Does Anybody know if this is a legitimate reason for an exchange?