The difference between him with KK or Galchenyuk, they are not good skater like him. They not protect puck like him. They not have physical ability like him.
Realistic projection for Slafkovsky is a 30g, 30a per season. But I think he can be a very good playmaker in this league, can reach 35-40a per season. He have great scoring instinct, great shot, so we can certainly hope that he can put 30 goals per season in NHL, maybe more, but if he put 30g, 35-40a per season with his style of play, that's a really good 1st overall pick.
Playing one day with Suzuki and Caufield, they are both pretty great playmaker too... that would help Slafkovsky to put goals.