Like I said, at any point in the season you could say "well so-and-so" isn't hitting but at the same time, you can't expect Jeter and Cano to have an OPS above 1.000 the rest of the year and Granderson won't have a .958 OPS. 5.7 runs per game is a pretty high estimation of what the Yankees offense is capable of.
The defense really hasn't improved much. They improved their LF defense, and that's it. And to be honest, Jeter's looked a lot like the Jeter of 2000-2008, and not like the 2009 Jeter who had the best defensive year of his career.
And like you said, the pitching will take a hit as well.
The Yankees are a good team, but like I said, they're not this good. They're definitely going to cool off from their current pace. Only a fool would judge a team by their April performance.