Excellent article about how good Julio is. Basically says he out-pitches his peripherals because he induces a lot of pop ups, and those high pitches are what lead to his higher than average HR rates. It then goes on to look at how pitchers that induced a lot of popups performed over their next few seasons. Turns out those guys did pretty well as a whole. Peavy was a close comparison.
The conclusion of the article is he isn't an ace, but he is a solid 2-3 with a great contract so should be extremely valuable.
http://www.fangraphs.com/blogs/is-julio-teheran-an-ace/