No, RorrKonn, Poser doesn't add polys -no change to the mesh- when using displacement, not on a poly-per-pixel basis.  Firefly temporarily creates micropolys for its own internal calculations of displacement.

     Yes, a single material can use maps of different pixel dimensions simultaneously.

     Qualified yes on the 8192 map;  current versions of Poser can use 8192x8192, but earlier versions are limited to 4096, if I remember correctly (correction welcome).  I can't remember if P9/Pro2012 could use 8192, but certainly Pro2014 can.

     And you can cheat to exceed that;  My envirodomes commonly are divided into four material zone quadrants to give me four times the resolution capacity.  You don't see it;  they're seamless transitions.  I use my rack-mount blades to network render those massive panos in Vue with spiffy atmospheres and high quality settings.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5