The one thing that might go for Gagner is the fact he is so young (although the same can be said for Voracek and Turris). Alzner, Ellerby and even Kane are all almost a year older than these three guys and that amount of time can make a big difference. Obviously Kane is getting drafted in the first few picks, but I don't think this will be lost on some scouts.
Jordon Staal was in a similar boat last year in that he was much younger than most of the other draft eligibles (almost a year younger than Kessel) and he went through a shocking level of development in the past year - in fact, he scored more goals in the NHL this season than he did in the OHL the year before. Obviously you won't see that very often, but I do think you will sometimes see the younger players develop a bit more, simply because they haven't had the time to develop as much when drafted. I still think he's likely to go anywhere from #5 to #8, but it wouldn't shock me to see him go a spot or two earlier.