It's kind of irrelevant, I mean you are going to have to kill current bosses eventually anyways. If you can do that you can certainly kill the T11 bosses.
Although honestly I don't really care about "proving you can kill the old bosses". For one, if you can kill the relevant bosses, who gives a damn about the irrelevant ones. Making you go back and do old bosses to get a new item is a pretty big contrivance too. I mean Blizzard has been trying to move away from stuff like that as it is.