Cain may be overrated, because even though he has enjoyed a lot of success in the Majors, he doesn't fit the typical mold of dominant pitcher, however, he's not a product of his park.
Over the last three years:
Home: 3.12 ERA.
Away: 3.42 ERA.
That's a perfectly acceptable split.
Also, it's a good idea to note that NL West is not entirely made up of pitcher's park. Petco and SF are pitcher's park, but Dodger (since they reduced foul lines) has played right around leagues average, and Arizona is a launching pad. He also doesn't only pitch in NL West stadiums.
Anyways, a lot of his success has to do with his career BABIP numbers (almost always below league average which suggests a trend), and ability to induce weak contact and keep the ball in the park (a 7.0 career HR/FB% for an extreme flyball pitcher is remarkable).