This is with best possible scenario, sync to vlank off in nvidia-settings and vsync disabled in game. Changing graphics settings ingame has absolutely no effect on FPS. I always ran DX8.1 mode on TF2 in Windows, simply because DX9 in TF2 is slow and ugly for me (especially as Sniper, I prefer DX8 clean scope over DX9 dirty blurry scope).
This is all with an nvidia 8400GS 1GB. Looking around the Steam for Linux discussions, I see a LOT of people are having the same problems with performance, even those with high-end CPU/GPU combos.