Idk about eye test part. I haven't really watched much of Guenther so i am not comparing, but in Slaf's case he looked good in his 2nd pro season. As others have mentioned his board play and ability to strip pucks is impressive. And his size definitely factors into the equation. Once he puts it all together in that frame he has serious potential to be a beast.
Important context though is Slaf is young. 20 years old still. Lots to work on obviously especially mentally (I want to see more confidence and assertiveness) but I like his trajectory a lot. He's shown me enough that he can become very good defensively one day. He has the tools for it.
100%.
He is very young. For his age, his defence is impressive. He certainly can become an effective defensive piece as he develops.
He's just not yet. Which is okay for him, it doesn't make him a bad player.
It's just when there is these claim that he is somehow a sure thing to become better defensively than Guenther, or that Guenther is not also capable of improving on his currently mediocre defense (and these claims/this line of thinking 100% comes from inherent bias about how people intertwine defense and physicality)