It is my opinion that taking an animal at any distance over 400 yards is not impossible, but it is not a great idea. I like to hunt for trophy animals and find it very dificult to judge the quality of any trophy out that far.
I have taken animals out past 400 yards, and every time I have been dissapointed in them. What looks to be very large through the binos or spotting scope, often turns out to have major ground shrinkage.
I think that T.V. has caused many hunters to think that they can take the long shot because they saw it done on T.V. The thing with television, though, is the shows are all edited and never show the misses and the wounded animals. I suspect that they also substitute beter quality animals for the closeup shots!
I guess it is really up to the individual, what are the risks you are willing to accept?
I like them closer than further!