while those are really complex boots as you say, I have a couple of suggestions on how to reduce polycount, first off a lot of that detail could be done with displacement such as the seams, creases, and even the rounded silver parts. getting rid of that detail alone could bring down your polycount by a third. also before exporting lower your smoothing rate in hexagon down to the minimum you can without causing too much roughness, remember that Poser tends to smooth far more then your modeler will show so you can get away with some roughness in the mesh. remember for each number higher the smoothing is set you are multipying the polycount by 4