From what I have seen from other people commenting on Poliigon and they all seem to think highly of it (other than Andrew Price’s tutorial freeloaders) but from what I have also seen is the quality of work that goes into making them and it is well worth the price of the membership, especially for someone that will need quality textures on a regular basis. As for Real Displacement I am not too sure on that one, just figured I would give some input on Poliigon and what I have seen and heard around the web from users of it.
There’s nothing that would prevent you from adding real displacement to Poliigon’s materials (the exception would be if Andrew didn’t provide any greyscale textures to use as the map).
There’s actually a couple of ways to use bumps with real displacement, one is setting the displacement mode to ‘both’ and the user is doing bumps via the normal input in the material nodes (using a bump map). On that, there’s nothing preventing you from modifying any of the materials either (as long as you don’t give them away on the forum).