It's certainly possible for him to add a couple of mph as he continues to grow, to fill out, and to master the craft. For young pitchers, developmental emphasis is primarily on repeatable mechanics: which is partly a function of teaching and learning how the body works (sort of like Han Solo talking to Chewbacca about repairing the Millennium Falcon: this goes here, that goes there), but also about developing the core strength to repeat balance point, downward plane (or "setting the angle") stride path, and landing. It sounds easy but doing it over and over again is hard work, and success is doing it with three or four pitches when you're a starter. And for a lefty it's a more difficult time achieving that consistency. So, yes, it's still possible for him to uptick. But, again, he doesn't need the uptick if he can spot it. That, too, however, is a function of mechanical mastery.
Again, I am not saying Owens is not a major leaguer or that he can't celling at the front of the rotation. I am saying that punching out AA hitters with a plus-plus change is not the same experience, and shouldn't be projected as the same experience, of dealing with major league hitters who you've got out get out three times a night. That change is only going to be as effective as the fastball that it must play off of. If you can't command the one, I am going to take the other away from you. That's how hitting at the big-league level works.
Plympy, I actually think Mellen has learned a lot in the last 12-16 months, for what that's worth. I have never much agreed with SP's rankings, as they seem as indiscriminate to me as old Rolling Stone music reviews. They're too volatile with young pitching -- What? Trey Ball getting knocked around? Oh dear! -- and not sufficiently mindful of (for lack of a better term) the larger prospect market against which all prospects acquire "value." I love Mookie, for instance. I can see Mookie being a dynamic player. But a lot of organizations have Mookies. Not many have Blake Swiharts.