The second unit if we're being honest could use someone who doesn't give up possession off the faceoff the majority of the time and it would be nice if Spezza never had to be caught out on a PK. Keep Kerfoot out there with him, he's pretty good at it. Outside of that it is still an upgrade in PK personnel.
Miller leads the Canucks in even strength points, he is their play driver. He has a similar amount to the likes of Marner, JT and Nylander.
Him being the only gritty player in the top-9 outside of Bunting is also a thing. He would lead the Leafs forwards in hits.
He basically does everything well. You're not going to get a comparable upgrade. That guy does everything.