Oh yes, that most certainly is a possibility, my only point with mentioning Gul'dan was to show that even a powerful mage such as Khadgar (most powerful mage in lore atm?) is afraid of him. It could mean many things, such as that Khadgar believes Gul'dan is more powerful than him or it is because he knows what Gul'dan is capable of and not really a direct result of their own powers compared.
In either case, the way I see it is that while Gul'dan could build a permanent portal to another world and also be used to power said portal. Khadgar is able to make temporary portals to another world in another timeline(powers we weren't even aware were available to others than the bronze flight) which makes them both forces worth fearing.
In the end, I think the legendary questline is leading towards Khadgar getting killed, by who or what I do not know but I get the feeling that Gul'dan will be the one left standing (at first).
I guess my point is, it's hard to tell which class is more powerful "in lore" because characters in lore are just as powerful as the story needs them to be.