I've had no distortion trouble with importing OBJs into Carrara and editing them in the Vertex modeler.  There are a few strange things, of course:

1:  When exporting a model from Poser as an OBJ, for some reason, I have to select the figure, select the body part (let's say it's the head), and then I have to "step out of Poser" (briefly open another -- ANY other -- program) and then back into Poser before I'm able to select the option that allows the head to be exported for a morph target.  I don't know if this is a Poser bug or a "Dex's computer" problem, but whatever the case is, it took me forever to figure this one out.

2:  After importing the model into Carrara (being sure to tick the make a single mesh for morph targets box), when opening the head in the vertex modeler, it's incredibly tiny.  Yes, you can zoom in, and that's fine, but the dolly/track controls become a bit unwieldy.  My understanding is that Poser's geometry reads as incredibly tiny in most other applications.  I remember seeing some third party product that would allow you to enlarge the geometry and then reduce it for ease of work.  Might be worth looking into.

3:  When exporting from Carrara, be sure to tick the "scale in local coordinates" (or something similar -- I'm going from memory here -- don't have Carrara or Poser in front of me right now).  Otherwise, your morph targets will get really wonky.

Hope that's helpful.

Have fun!

:D