"The Poser subdivision can be set to "0" for preview and "1" or "2" for rendering, so you can have a superfast OpenGL preview but still a smooth render without the need to switch subD levels inbetween."

You can do the same with the DSON for Poser Sub-Division, in fact setting it to -1 turns off Sub-D and gives you the cage for preview.

"The HD morphs don't work with the Poser SubD, though."

Neither does edge creasing from the Open Sub-D Spec, at least as I have been able to determine. (Both HD morphs and Edge creasing do work with the DSON Plug-In SubD if you have a model that uses it.)