Even though I'm maybe his biggest fan (except his Mom probably) I don't expect sub 2 ERAs long term.... but low to mid 2 ERA, is very possible. The scariest thing is, it's when he hits his fatigue wall when he gives up nearly all his runs, when he builds up his endurance and if the Angels improve their middle relief options, he might actually be a sub 2 guy, but at some point, I presume the hitters will manage some adjustments, but there's not a lot of adjusting you can do to location/change of speed pitchers with his wide variety of pitches he can locate.
From a hitter's perspective, what makes him so tough is, it's just impossible to know with any certainty what he's going to throw, where he'll locate it, and at what speed it will be. I really think that's why the scouts didn't predict how good he'd be so fast. He's doing the same thing to the Yankees that he did in college, but in college, the scouts could always go, well, this is just college, the kid doesn't have a killer pitch. But if you're a hitter that doesn't have a clue what pitch is comng, a Yankee is about equal to a college player.