I've been pondering that I may be over supplementing vitamin E. I've seen recommendations of 400 IU per day which is pretty high (as per the articles linked in the OP).
Since vitamin E is fat soluble I know one should be more careful.
I ran across this article written by veterinarians about vitamin E in racing sled dogs. I noted that the 400 IU was for active racing sled dogs, not average household dogs so I thought I'd share this for informational purposes.
Supplementing Vitamin E to Alaskan Sled Dogs – Iditarod