That higher ceiling speculation is indeed little bit weird. If Hughes' achievements have been exceptional for several years now, so have been Kakko's, and if you use perceived historical trajectory over several seasons to make an argument to solidify other player's projected ceiling, then you've to apply that same argument also to other player. Their age difference is negligible.
Kakko's trail have been made against older guys over his entire hockey career, and at this point, nothing, nothing indicate he has reached his ceiling, and if we look at his floor against NHL-players... lol. Of course after 3 games, it maybe just hot streak, but even if so, that doesn't change the big picture.
Kaapo is just continuing his overall career trajectory now. Trajectory that by all measures has been far above and beyond what can be considered as average norm for Finnish prospects even during these high-tide years of production quality of Finnish prospects.
Biggest disparity between them is just NA news coverage, in which I assume Hughes will be always in uphill position over Kakko.