It's not hard to see that McKay will get drafted just on his size and length. Needing to add weight is like the least important thing in the entire world when it comes to getting drafted in the NBA. Anyone can gain weight. Especially with world class nutritionists and training.
Look at Noah Vonleh.
6'8 without shoes. 7'4.5 wingspan.
McKay is probably 6'8-6'9 without shoes, 7'4 wingspan.
Vonleh was 247 pounds at the draft combine and was listed at 240 by Indiana.
McKay is listed at 215, which is obviously low. But look at Vonleh's weight just two years before and he is listed at 222. He put on 25 pounds in two years and at most grew a half inch.
http://www.draftexpress.com/profile/Noah-Vonleh-6471/
Now compare their stats per 40 minutes and you get:
McKay 17.8 points/10 rebounds/1.1 assists/4 blocks
Vonleh 17.0 points/13.6 rebounds/.9 assists/2.1 blocks
McKay is shooting 60.8% from the field
Vonleh shot 52.3% from the field
Vonleh did make a 3 every other game or so but was not as good of a shot blocker as McKay is. And this is all with McKay playing almost exclusively in the Big 12 without playing in the soft part of the non conference schedule.
And Vonleh went 5th overall.
So my point is that even if Vonleh is better than McKay, you can at least compare him to the #5 overall pick. That tells me that he has a pretty damn good shot to get drafted.