<ParticleDetail>2</ParticleDetail>
<UITextureDetail>2</UITextureDetail>
<LevelTextureDetail>2</LevelTextureDetail>
<CharacterTextureDetail>2</CharacterTextureDetail>
<EffectTextureDetail>2</EffectTextureDetail>
<ZBufferBits>24</ZBufferBits>
Changing stuff like this might help. Lower number = lower quality = better performance. I used to have GhR tweaked to the max on my iMac 800 to make it run decently. I had all details on low or off (except shadows, you might spot someone through a wall with it
And also turn bullet holes to 1.
That's all I can think off right now.