I think you underestimate just how bad Detroit's infield defense has been. Over the past 2 seasons, the infield defense has cost the Tigers 52 runs. Spreading those runs out over the starting rotation, Porcello's Defense-Adjusted ERA over the past 2 seasons would be 3.92 in 2013 and 3.03 in 2014.
In addition to better defenders, Porcello should also be helped out by the Red Sox' defensive shifting. Detroit is a team that has lagged behind in that regard.
Better pitch framing should also help Porcello. Avila was not very good at framing. Hanigan will not be as good as Vazquez, but he is still very strong in that area. Each extra strike is worth approximately .14 runs.
Playing at Fenway is not going to offset all that defensive advantage, especially since Porcello is a ground ball pitcher. He gives up more HRs to left handers, and will probably be helped out by playing in Fenway with its deep right field. Fenway has the 4th lowest HR index for left handed hitters, while Comerica is neutral. Porcello's high home run rate so far is more or less due to random variation, or flukiness.
http://www.fangraphs.com/fantasy/three-things-the-red-sox-will-do-for-rick-porcello/