DarkEdge, I'm a big fan of using both maps on a model. 8K maps are the lower end of feature film res - if you're exporting maps that large and not getting what you want, I think you may need to look at how you're building up the displacement and at the topology you're applying the map to.

You need to build up your displacement. I see folks going to the highest subdivsion and starting to sculpt. that's great and all, if you're not exporting a map but if you are...you're missing the point. You build up form on the lower levels of subdivision, adding detail as you move up one level at a time. The highest level of subdivsion should be left to things like scratches and pores. This will give the app more to draw from (it looks at all levels of subdivision for building the map).  Once I got that in my head, I've never not been satisfied with a map out of ZBrush and I don't render maps higher than 4096x4096.  

Another huge factor is the detail level of the mesh in question. Does it contain enough polys flowing in the proper direction to support the detail you're trying to put on it? This is why folks retopo meshes - to support the shapes and detail they are trying to achieve. Sometimes you run into a flow that just does not want to do what you want it to do - you either have to accomodate your design to the flow of the mesh or you have to accomodate the mesh to the design in your head. Either way, something has to give or you're going to run into areas that look...sloppy. 

Last thing to look at is how the app handles displacement/normal mapping and what other effects you're using. Some handle it better than others, there's not if ands or buts about that. Poser does a decent job of it from what I've seen, so getting good results out of it should be entirely possible. If you're using SSS with displacement and normal maps keep in mind it has a softening affect to the overall look and consequently, some details may be lost, requiring you to crank up or turn down the values a little where possible.

That's my two bits. 

OH! one tip: Poser supports 16 bit TIFF files. So if exporting out displacement maps, you'll want to use that option. It will hold more info than its 8-bit counterparts, resulting in more detail/better looking displacement.