I know FX makes a big difference in terms of DOF, so this was an area I was curious for using FX for. Good question to ask you though, do you know why Pros usually suggest using FX for portraits? I have never discovered the reason why.
Your remark about DOF points to one argument for the larger format. With any particular focal length the field of view is wider on FX than on DX. This wider FOV requires the photographer be closer to his subject so as to get the image framing he is seeking. A shorter distance between the object being photographed and the imaging surface means a shallower depth of field. This hold true for any lens of any focal length - the closer you are, the shallower the DOF. The larger the format - the wider the field of view, an 80 mm lens is a telephoto on DX, a short telephoto/portrait on FX, normal on MF, wide angle on 8X10 view.