To be clear, same slow downs and same difference in brightness in landscape or portrait, however I do run at 2560x1440 since that's the resolution of the workstation's monitor. I'll test the HD 6950 on the 1080p TV this evening once I'm home from work.
Ok, I was a little surprised, because we have the same GPU and your CPU is better than mine.
But we have a difference. My back buffer is "only" 1080x1920.
I get slowdowns in portrait mode when I use your resolution (or Eldars of course ).