Of course running in DX11 mode causes a performance drop, but what about just INSTALLING DX11 patch? As many performance problems came after 1.9 + DX11 patch, I was wondering if reinstalling the game without installing DX11 patch would do any good to my performance (remember I have a weak setup, E8400 C2D and ATI 5770). I tried the CPU affinity thing and I didn't notice any significant improvement.

Opinions?

I've got a GTX260 (basically similar performance to the 5770, slightly faster but only support for DX10) and I didn't notice any performance drop from 1.8 to 1.9 and I run on Ultra (DX9)

I've got a GTX260 (basically similar performance to the 5770, slightly faster but only support for DX10) and I didn't notice any performance drop from 1.8 to 1.9 and I run on Ultra (DX9)
I see you have a C2Quad, maybe that helps, or Nvidia cards are less affected...

Well, in this circumstance, Nvidia are actually better supported to the likes of the engine, so a majority of nvidia cards will perform better than ATI.