Everyone's saying that Zacha 'Has the potential to be a top goal scorer" but the fact is that he has yet to prove that even in junior let alone show that he can do it at the NHL level. Sure, I do think he can be effective but there's nothing yet solid that gives any indication that he can be a 30 goal scorer in the NHL...he could just as likely turn out to be Zubrus.
With Barzal, he's shown both in the WHL and on the international stage against his peers that he can be an effective playmaker. Yes, he has a good shot and no, he doesn't use it nearly enough but I'd say it's more likely that he turns into at least a 15 goal, 60 point center based simply on what's he's shown thus far.
You're contradicting yourself here, making the argument moot.
McDavid can be Dainius Zubrus. Eichel can be Ville Leino. However, with the tools they have demonstrated on amateur levels, they are more likely to generate numbers (after a season+) closer to the top NHLers of today.