High HR/FB isn't a warning sign, it's an indicator of bad luck, just as low BABIP is an indicator of good luck. In his case I would say it is very likely that he overperformed in terms of ERA last season, but not by a large margin. OTOH, I don't think he pitched up to his potential last year. It took him years to get adjusted to being a Major League pitcher, and once he got that he was almost immediately cast as an ace. I think that took some adjustment as well. To be totally honest, he's not the strongest player psychologically. I think that's pretty obvious by this point. However, it's worth pointing out that in the second half of last season Tillman was appreciably better than he was in the first half. His record dropped off, but his ERA, WHIP, K rate, and walk rate all got better as the season went on. If you dig into his PitchFX database, you'll find that from late June on he threw substantially more strikes and drew substantially more swing-and-misses than he did through the first 3 months of the season. He's also throwing 2-3 MPH harder than he was this time last year, although not as hard as he was later in the year, particularly in the warmer part of summer.
All things considered, having watched the guy pitched, I have a high degree of confidence that he's a better pitcher than last year's stats would suggest. Possibly a lot better. I wouldn't trade Lester or Buchholz for him, or Ryu, or anybody anywhere near the top of Washington's rotation. But he is incredibly deceptive, since there just aren't many over-the-top pitchers anymore, and nobody knows what to do with a true 12-6 curve coming from a high arm slot. I think this year you'll see an ERA in the vicinity of 3.3-3.4 from him, which is a pretty solid number in the AL BEast.