Correct me if I'm wrong but...
I believe the medium slider setting does not use the "_N.dds" or the "_S.dds files (to increase FPS). When the Slider is set to highest the afore mentioned
files are used. Which leads me to suspect poorly created "Normal.dds" and or "Specular.dds" files. (Wrong settings used during creation).
Windows pro 8.1 with 8.1 update,64 bit|AMD FX-6200 Six-core-3.80 Ghz |CPU PASSMARK 6,142 |Memory 10GB |NVIDIA GEFORCE GTX 660 SC |GPU PASSMARK 4,114