I would easily sign Mack to an 8x8 contract right now and call it a day. It probably doesn't happen because of the contracts Duchene, Landy, EJ, and Varly are all on right now, but I would do it if it were possible.
Sure Mack isn't worth the 8M right now, but he absolutely will be worth the 8M and more in a couple of years from now.
As someone said earlier Mack is going to be an 85-90 point player in his prime years. Players who produce like that are going to start getting paid 10-12M in free agency, its already started with the Kane and Toews contracts and will continue with Stamkos' next contract.
Mack at 8M would be one of the best contracts in the league by year 2 of the deal.