I also experience unreliable gameplay performance with the AMD Radeon HD 6790 on Ubuntu 12.04. It is mainly with the game Left 4 Dead 2 that I've tried it so far. I get stuttery performance during the map intro scenes (both video and audio), as well as intermittent input lag during the gameplay itself. Sometimes, during an online game, I will press left-mouse button to shoot, and it will start firing one or two seconds later.
Up until one week ago, I was using the Nvidia GT 240 graphics card (1GB GDDR5), with the latest stable-release Linux drivers. I experienced no such performance lag.
I play with Vsync on.
Hopefully this will be addressed in upcoming driver releases. However, thank you for your efforts thus far, AMD Linux team.
Ubuntu 12.04.3, 64-bit
Unity Desktop Environment (vanilla)
Asus M4A78LT-M LE Motherboard
Asus BIOS Version 0803
AMD Athlon II X4 645, 3.10 GHz
AMD Radeon HD 6790
Catalyst Control Center Version 2.18
ATI/AMD Proprietary FGLRX Graphics Driver (Post-Release Updates)
AMD should start treating their Linux customers a little more seriously. This is AMD customer support response to my ticket, ticket in which I've provided same information as in this thread along with Xorg logs:
What the hell? No wonder AMD Linux drivers work so bad.