The 165/2.8 is noticeably brighter and easier to focus than the 200/4. Wide open, it's sharp enough, but DOF at portrait distances is so shallow that you can get an eyebrow in focus, while the eye is out of focus. Even though you'll probably shoot portraits at f5.6 or smaller, the f2.8 maximum aperture makes the lens easier and more pleasant to use.
As for it being not long enough for tight head shots, with the larger format, there's less need to crop so tight. To get the full benefit of the big neg, you'll be making big prints, and the subject will appear large enough without tight cropping. Bruce Dayton's excellent PUG photo is a good example of this. Pat White
