I'm kind of with Handlogten's Heros at this point regarding Mirotic. He might be undervalued at this point. I don't think his defense is really worse than Harris's. The need for Harris's playmaking is somewhat obviated by the presence of Conley.
But I'm still not moving on from Favors just to get Mirotic. Favors may also be helped greatly by Conley's presence. If you could get Mirotic and a very good backup center, I'd think about it. If you could get Mirotic, a decent backup C/PF with some defensive chops (say Vonleh), and a shooter (Ellington, Bullock) with Favors's money, I'd take a very long look. But I think we're likely $10M short for that.
I don't want to replace Favors with some poor defenders (Portis, J. Green) who seem like they can shoot (I think Mirotic is likely to be a better defender than them). If you're getting them, it better be because you have some really good defense coming back as well or you're just keeping Favors (though it sounds like the money will not add up).
So I'm entirely happy with going into the season with Favors and seeing how things turn out. Free agency will be interesting, but there's a good chance that we can't find anything that will make it worth moving on from him.