Same problems here with bad/terrible performance... but the latest (13.11) beta is abit better than before. I have 2 monitors and on the last driver I could not use them at the same time because it was so terribly sluggish. Forget fullscreen movies or playing anything i flash whatsoever.
I've sort of just (1,5 months ago) switched from being a Windows-nerd to Linux-geek (sooo much better) but I could never imagine that the Linux-drivers was this terrible.
I'm sitting on Fedora 19.. since the card (driver) was unusable in Fedora 20 (Wayland).. but I'm probably switching to SteamOS (beta) any day now. My hope is that this mess can be cleared/helped by having Valve pulling a string or two..
CPU: INTEL CORE I7 4770K 3.5GHZ | MB: ASUS Z87-A Z87 S-1150 ATX | KINGSTON 2x8GB 1600MHZ DDR3 | Sapphire Radeon R9 290