Linus at LinusTechTips said at release of 290x that he already had a waterblock ordered from Koolance, I'm sure he will have a video of its performance compared to a watercooled 780 in the coming couple of days.
I snagged a pair of Sapphire r9 290x cards on Newegg last week (got really lucky) -- they just arrived in the mail today. Woohoo! I'm having a tough time finding the water blocks though. Out of stock everywhere (both Koolance and EK brands). I checked all the resellers listed on the Koolance website. No dice. (backorder/ETA is 12/27/13 through Koolance).