Remember that pipeline is moving a product through the steps for finishing. Great tools like Zbrush, Modo, etc. are helpful at many steps of the pipeline. One of the charms of doing it yourself is you have the opportunity to tweak what you've done in previous steps of the pipeline rather than in a formal shop where the mesh gets rejected and sent back to you for "corrections". ;)

So if my dragon's body tool UV settings were at 8k res, will it import the maps to Poser that size?

Unless you re-scale the output maps, then yes. Bear in mind that map size use's memory at the square of the size (that is, double the size of a map on a side and you use four times as much memory) so there is a tradeoff. You want to make sure that you're making good use of the map space at 8K.

My thinking to just assign materials based on my current ZB textures should just work straightforwardly then?

You may have to flip them in Photoshop. I can never remember whether ZB's default UV space matches Posers...