I don't think we are overachieving because if you look at what we have done so far, it is basically win the games we should win. And we won at San Antonio, which essentially made up for the blowout loss at Houston.
Being a great NBA team is largely about winning the games you ought to win, which means the VAST majority of your home games and road games against inferior teams (teams significantly worse than you). So the only aberrations there really are Houston (bad loss, though not an easy game, especially at the end of a road trip) and San Antonio (great win). Also, losing the second game of back to backs on the road are always understandable.
Plainly put, we are focused and taking care of business rather than overachieving. If you look at us individually, no one is having a particularly great season, or a season they are not capable of. Yes, Melo is playing very well, but it's not like he has not always been a great scorer.
If I had to name the biggest key for our early success, it's taking care of the ball, and I definitely think this is a sustainable thing.
There's definitely room for improvement. Namely, I'd like to see us rebound better. I think the offense will even out a bit in terms of fewer 3-pointers when we get Amare and Shumpert back.
So overall, I am obviously very pleased and surprised, but I just think we finally have a coach who demands focus and attention to detail, and we are seeing our talent and experience prevail over lesser teams because of this.