A harsh comment, considering one of your favourite players to line up with McDavid and have success was Maroon. A player whose skating is comparable to an oil tanker on skates. Patrick (and Neal, when healthy) showed that even an average level of skating isn't a prerequisite to succeed with McDavid: Smarts go a lot a further.
This isn't to say Benson would have success, but just highlight that footspeed isn't a requirement to play with McDavid. AA also proves the opposite end of this spectrum (speed with McDavid = $$) isn't true.
McLeod with McDavid sounds a lot like a worse version of Nuge with McDavid; Not bad, but you're adding a player, who's main talent is transporting the puck from a to b, to the player who needs the least help in the league doing that.
Plus I don't see McLeod having the vision, shot, or creativity to generate chances from the room that McDavid gives him, he'd just defer back to Connor. He's also not strong along the boards, easily getting out-leveraged; which if you have McDavid controlling the open ice, you better be good at. Just always being the high guy back, allowing McDavid to attack more isn't enough.
This isn't a shot at McLeod, but it's just not his game, even dating back to juniors. Even last year in the AHL, a big part of his success was being the guy that legged the puck up the ice with insane efficiency, drawing defenders to him before deferring to a wide open Marody or Benson in the offensive zone (players that needed that help getting the puck from a to b).
I like really like McLeod, but playing with McDavid isn't the role for him. McLeod's value is the ability to control open ice, and move the puck forward, getting it to open teammates. He's a Todd Marchant 2.0, or a more contemporary comparison: a forward version of Nurse with the puck.