I am also one of Porcello's biggest supporters since day one, and without the benefit of hindsight, I would give him that extension again in a heartbeat.
I'm no expert, but I think he is trying too hard rather than just pitching. Someone pointed out that he's throwing his sinker too hard, and it's not sinking. He says he knows what the problem is, but he hasn't been able to fix it.
Perhaps a couple weeks off (phantom DL) to correct his problem would help. If I'm not mistaken, Porcello was pretty good after coming back from the DL in 2015.
On a somewhat optimistic note, his K/W ratio still looks pretty good.