I think he'd be a significant upgrade for them, particularly with Wade's health up in the air and him likely missing games next season with his maintenance plan. Lowry would give them another guy that can create his own shot and take pressure off of Lebron. He's a decent enough shooter from 3. His biggest impact would probably be on the defensive end. I don't know if Miami will be able to sure up their interior defense, so adding a PG that can really get after opposing guards would probably be their next best option.
Don't know if it will come to pass, but Lowry to Heat would be a great move for the team.