I've been using the Quixel Suite lately, and I'm pretty happy with it. When working with lowpoly models that have normal maps baked from their highpoly version it works nicely (though I've had some issues working with the preset smart materials, but with some small adjustments they do their job).
What happens is that I'm currently working in a short film in which most of the props don't have a lowpoly version, they only exist in their highpoly version.
Checking the Quixel's Wiki, it says that the main difference for working with highpoly models is that we have to provide a Curvature map to Quixel, as in the tangent normal map there's not a lot of information about the model's shape, so Quixel can't figure it out (this is what I understood).
I've tried several ways of creating Curvature maps: from Blender using the dirt vertex coloring, from XNormal (monochrome and colored versions), and finally, as you say it's the quickest way to do it, by processing the Ambient Occlusion in Photoshop.
So far, none of these methods worked.
Can you post some sample file of a highpoly model test to see how the textures look? Or post a curvature map that Quixel needs to see how it looks like?