You can change the Visuals only with Tools like SweetFX, ENB Series or Dark3D.
That won't change the fact that you would need better Textures and change them which would take time (Ground Textures, Model Textures, Skybox Textures and Object Textures)
Even if you did that, these low poly Models (Speaking of a NPC with 400-600 Polygons) would look horrible.
The only thing you can add into SRO via these Tools is SSAO, DoF, AA (Text gets distorted), better Bloom at night and day cycles and ya.
The only thing I found are some Shader Files (not sure if these were in the Data.PK2) while I tried to add the new Reflection System from SRO-R into the vSRO Client, maybe they have some use...