Here's a couple of things i've tested out on a 6800GT (pci-e, 256mb) that seem to have improved performance. Naturally, you should ALWAYS backup the files before you modify them.

Remove Intro Movies:
Go HERE, download and apply

Game Settings:

• Rename the 2 files in \Engine\Shaders\HLSL\StaticProjectShadow\
  
• Open ..\Engine\Shaders\HLSL\MegaPostProcess\Blur.hlsl in Notepad and change the line that reads:
  return lerp(PreviousColor, BlurColor, Factor) + BloomColor;
  to:
  return lerp(PreviousColor, BlurColor, Factor);
  
• Open ..\KellerGame\Config\PC\KellerEngine.ini in Notepad and change the following lines to the settings listed
  UseTrilinear=False
  LevelOfAnisotropy=2
  ( or you could use True & 1, just test and see what works best for you)
  
  UserMaxTextureSize=2048
  ShadowBufferResolution=512
  (Shadow resolution. Will look blocky but your framerate will improve.)
  
  ShadowBufferResolution=512
  MaxShadowResolution=512
  MinShadowResolution=16
  BlurBufferResolutionDivisor=1 // 1 = Low, 2 = Med, 3 = High
  MinStreamedInMips=64
  MaxStreamedInMips=128
  (Or try 128/512 if you like. Test to see what works best for you)
  
  
• Open ..\Engine\Shaders\HLSL\ShadowDepthTest.hlsl in Notepad and change
  clip( (0.5f - SceneDepthValue.b) * NeedStencilTest );
  to:
  //clip( (0.5f - SceneDepthValue.b) * NeedStencilTest );
  
  This line actually occurs 3 times in the file. But, the one you'll be concerned with is the one in either the nvPixelShader or atiPixelShader function depending on whether you have a Nvidia or ATI card.
  
• Open ..\Engine\Config\BaseEngine.ini in notepad and change
  MinStreamedInMips=6
  MaxStreamedInMips=14
  
  Change to:
  
  MinStreamedInMips=64
  MaxStreamedInMips=128
  (Or try 128/512 if you like. Test to see whatworks best for you)
  
  BlurBufferResolutionDivisor=1 // 1 = Low, 2 = Med, 3 = High

Sound Tweaks:
(I haven't tried these as yet, but thought I'd post what I'd found so some of you can try them out)

• Open.. \KellerGame\Config\PC\KellerEngine.ini in Notepad
  Find
  [ALAudio.ALAudioDevice]
  MaxChannels=32
  TimeBetweenHWUpdates=15
  
  change to
  
  [ALAudio.ALAudioDevice]
  MaxChannels=64
  TimeBetweenHWUpdates=10
  
• Open ..\Binaries\DARE.ini in Notepad
  # for Sound Blaster Live! Value and equivalent sound cards:
  
  [Renderer DS3D Options]
  DS3D_CACHE_SIZE=6291456
  [Sound Manager Options]
  NB_AUDIBLE_VOICES=32
  [Renderer options]
  Output frequency=11025
  
  # For Audigy 1 Series and equivalent cards:
  
  [Renderer DS3D Options]
  DS3D_CACHE_SIZE=8388608
  [Sound Manager Options]
  NB_AUDIBLE_VOICES=32 **OR 48, depending on your system...try both**
  [Renderer options]
  Output frequency=44100
  
  # For Audigy Series 2 and equivalent:
  
  [Renderer DS3D Options]
  DS3D_CACHE_SIZE=8388608
  [Sound Manager Options]
  NB_AUDIBLE_VOICES=64
  [Renderer options]
  Output frequency=44100

This guide has been copied (and tidied up for easier use) from http://forums.ubi.com/eve/forums/a/tpc/f/3801065024/m/8361088135

Please post back any results (positive or negative) and let us know how you go.

NOTE: REMEMBER TO ALWAYS BACKUP YOUR FILES AND/OR FOLDERS BEFORE MAKING MODIFICATIONS. No responsibility is taken for you being a n00b.