If you want to risk it, it's your card. I'd rather keep my warranty rather than give them any excuse to deny.
I did. I like my computers quiet. Ever since the rise of the tower heatsinks it's been very easy to cool CPUs quietly. Not so much with the videocards, they put out as much heat as the CPU now, but the heatsinks are tiny, so it's much harder to cool them quietly. The stock cooling solutions, even tri-fan ones always sound like a jet engine to me. I have used aftermarket coolers on every video card I owned since 6800GT. The only card that really failed on me after installing aftermarket cooler was EVGA GTX260c216. It put it back to stock and it was RMA'ed without any problem. I put the same aftermarket cooler on warranty replacement and it's been chugging along just fine for the next few years until I finally retired it.
The AIO closed water loop "mod" works really great for video cards. Mine never goes above 65 degrees under full load. That's a lot cooler than stock, and a lot quieter too, quieter than even these tri-x cards. I have also had problems with stock fans failing because lubrication dries up from the extreme temperatures they're subjected to. What do you do then? Replace your card through warranty? That's not an issue with the mod because the temperatures are not as extreme and even if it does fail, it's an easy replacements - you just pop in another 120mm fan.
Point is, the "mod" is great, easy to do, and cools better than any "tri-x" solution ever will. If anyone is really paranoid about warranty, just buy from mod friendly company such as XFX.
So the Sapphire one has Hynix, Asus and Gigabyte has Elpida?
Anyone know how the XFX R290 DD stack up? I can't find any reviews on it.
From what I understand the memory you get is a luck of draw. It depends on the manufacturer, but AFAIK when it comes to R9 you're most likely to get elpida, followed by hynix. I don't think I've ever seen report of R9 card with samsung ram, but I do not really follow the news too closely.