I suppose as with anything it’s down to opinion, but I absolutely do not see “good second line” potential in Gushchin or “good second pairing D” in LSW or Cagnoni at all. I see Gushchin as “okay 2nd liner on terrible team” or “solid 3rd liner on a team specifically constructed to allow their third line softer minutes”. I think a LOT of things would have to go right for either LSW or Cagnoni to even make the NHL, I don’t see any way either can be a true top-4 D. Small left-handed defensemen with non-elite skating and non-elite skill don’t tend to become top-4 on good teams.
I'm more optimistic about Sahlin Wallenius, though I get the concerns about his upside. But I see a guy who's more mid-sized than small (6 feet with room to fill out), and while calling his skating elite might be a bit rich, everything I saw said he was one of the best skaters in the draft.
And while he did fall into the Sharks' laps later in the 2nd round, he was frequently seen as a guy with late-first potential. Anyway, I might just be a little biased because I love a good-skating d-man, and I have a soft spot for someone who gets dinged because he's just (just!) a complete player with no true standout/splashy tools.
I'm not going to expect more than a 3rd pairing guy, but with that complete package, good skating, and supposedly good smarts/vision, I don't see why his ceiling would't be as a 2nd pairing d-man.