Mule Deer, I remember meeting you in Wyoming at dinner one night in what may have been 2000 or 2001, I was there hunting Pronghorn with my outfitter Jay Lesser, you were with the Swarovski bunch, I think Jim what's his name was your host.  We also toured Talley's facility when they were there, Gary is a friend and had invited us to look around.  Anyway, I remember John Lazzeroni was there "product testing" a few of his rifles, he was pitching Franz, the Swarovski family member who I believe represents their crystal business in the US.  John seemed to be more interested in getting as far away from the antelope to make the shots as long as possible, I think he ended up at 650 yards or something like that.  I killed mine at something like 250 and that was far enough for me.  
I typically hunt in the south, most often attached to a pine tree 20 feet off of the ground.  300 yards can seem like a very far distance with an imperfect rest and tree swaying in the wind.  I guess I would say knowing your gear, having a decent rest, knowing the distance and having a target that is not on the move would be the determining factor for me.  I have several 1,000 yard rifles but consider my max to be about 400 yards on big game, much prefer under 200.