I think that we have great Beorn model ...
although he is one of the beornings.
Yes, as I wrote earlier, this model is really nice. As you said, it is model for Beornings right now. I am for little modification of that model (add tunic, belt, new unique axe as weapon, dispense of cape), so he will look similar to this one:
This look fits description from book almost 100%, that is reason I think it is best.
Also Grimbeorn has different kind of axe (relative to other Beornings), new Beorn model also deserves unique model of weapon=axe.
Edit:
News from modb.com, Grimbeorn's bear form (
I presume ).
Many thanks to Edain Graphic department!