More information about PTex is available available here
PTex files are strongly connected to the topology of an objects's mesh. Therefore any modifiers which change the topology (including TurboSmooth and MeshSmooth) will break the rendering of the PTex texture. If you want to smooth the object, you can use the Displacement Modifier | GeomDisplacedMesh modifier with Subdivision Type and 0.0 for Displacement amount.
When using PTex textures for displacement, small cracks might appear along polygon edges because of slightly different filtering applied to the PTex texture on adjacent polygons. Setting the filter width to 0.0 will improve the situation, but will cause the most detailed mip-map level of the PTex file to be loaded thus increasing the required memory.
When using PTex textures for displacement, it might be necessary to adjust the min_bound and max_bound parameters of the GeomDisplacedMesh modifier, especially if the texture has negative colors. Otherwise the texture values might be clipped away.
