So one of the first things I would recommend doing to the game is using Ambient Occlusion.
For Nvidia users go to your control panel to activate(or use nvidia inspector to force it), for AMD get RadeonPro(http://www.radeonpro.info/download/). For people with older hardware or Intel graphics chips use EnbSeries.
For Thief Specifically use Enbdev.com
or the injector Enbdev.com
I would also try this for texture replacement dxtool
In the mean time I will also be looking into trying to get some more people on board to get high poly models so that hardware tessellation could be enabled for Ati users, given that DX9 to my best knowledge does not support tessellation.