Looking around for Arkham Knight tweaks, I found the this guide the most helpful.
Just to give you a reference point, I have an Intel i7 950 and an AMD Radeon R9 290, and most notably, the following tweaks seem to have helped me the most:
1. Texture Streaming
I've found this to generally be helpful, but I've only tried this on a WD Black and an SSD - your mileage may vary.
[TextureStreaming] MinTextureResidentMipCount=7 PoolSize=4096 MemoryMargin=192 MemoryLoss=0 HysteresisLimit=11 DropMipLevelsLimit=16 StopIncreasingLimit=12 StopStreamingLimit=8 MinEvictSize=10 MinFudgeFactor=1 FudgeFactorIncreaseRateOfChange=0.5 FudgeFactorDecreaseRateOfChange=-0.4 MinRequestedMipsToConsider=11 MinTimeToGuaranteeMinMipCount=2 MaxTimeToGuaranteeMinMipCount=5 UseTextureFileCache=False LoadMapTimeLimit=5.0 LightmapStreamingFactor=0.1 ShadowmapStreamingFactor=0.3 MaxLightmapRadius=2000.0 AllowStreamingLightmaps=False TextureFileCacheBulkDataAlignment=1 UsePriorityStreaming=True bAllowSwitchingStreamingSystem=False UseDynamicStreaming=True bEnableAsyncDefrag=True bEnableAsyncReallocation=True MaxDefragRelocations=256 MaxDefragDownShift=128 BoostPlayerTextures=6.0 TemporalAAMemoryReserve=4.0 ReflectionTexturePoolSize=96
2. Upping Maximum Framerate
As my computer couldn't maintain 60fps, I chose to only increase the maximum frame rate.
[SystemSettings] ... MaxFPS=60 ...
[Engine.Engine] ... bSmoothFrameRate=TRUE MinSmoothedFrameRate=29 MaxSmoothedFrameRate=62 [Engine.Client] ... ; This is actually your refresh rate, set it to match your desktop's refresh rate and ignore what its name seems to imply MinDesiredFrameRate=60.0
3. Motion Blur and Reflections
I found that turning off Motion Blur and Reflections also helped with frame rates, especially during rain, it does make your cape and the Batmobile less shiny, but sacrifices have to be made.
... MaxFilterBlurSampleCount=1 ... Reflections=False AllowImageReflections=False AllowImageReflectionShadowing=False ...
4. Update Drivers
This actually helped a lot - updating to 15.6 beta (now you can get 15.7.)
Hopefully this helped you get to roughly where I got to at the moment, which is about 60 fps with occasional dips to 30 fps. (I suspect some of that is due to my now 4-year-old CPU.)