I like your process for evaluation. Stats should indeed be used to supplement what your eyes tell you (assuming you watch enough of the particular player).
I do think the stats are "clearly wrong" with Arrieta in regard to him being "unlucky" or due for improvement, but I also think they indicate that he still has potential. Improvements in strikeout/walk rates are always a good thing and in the very least could portend a successful transition to the bullpen. The consensus seems to be that those of us who hold out hope for Arrieta becoming a successful SP are hoping that he fixes his mental issues in order to let his physical abilities play out. I personally wouldn't say that is likely to happen but I will continue to hold onto a sliver of hope that it does. That being said I completely understand birdwatcher and Matt's point of view that the Arrieta love is "baffling". Time will tell if some of our continued hope and faith is rewarded.
I think he will be on a short leash if/when he reaches the majors in 2013, a couple "more of the same" unraveling bad starts and that may be the last we see of him in an O's uniform (at the very least as a SP).
I agree with what you are saying. I even went batter by batter in a few games that people claimed he was "unlucky" in to show that he wasn't last season. I am firmly in the camp that believes his issues are 100% mental. He falls behind hitters and I think that is his main problem as he doesn't trust his stuff. Then, when the hitter is in a hitters count they are at such an advantage and that is why he continues to struggle. He gets away with it from time to time because his stuff is very good but nobody can make a living pitching like that. Talent wise he is clearly ahead of Matusz and Tillman in my mind but mentally he is very far behind.