Esteban showed visible improvement towards the end of the season compared to Hulkenberg.
Not really. That was probably an improvement of the car what some people see as an improvement of Gutierrez.
Esteban was the best compared to Hulk between Spain and Belgium, where he finished practically all races withing 10-15 secs to Nico. In the end of the season, the gap between them has widened a bit, Esteban was a bit more inconsistent when the car became more competitive.
For me, he was certainly more impressive mid-season, when he was massively underrated and overinsulted, than at the end of the season, when some people came to the conclusion he is not that bad, but actually, he was not as good as in previous races.
I still think he is more talented than Van der Garde or Leimer (beat them in GP2 despite inferior experience), so I'm happy some people's wish (maybe not that visible on this board, but it's a popular wish in some other places) that he would lose that seat for one of these two didn't come true.
Sutil definetely has much to lose in this battle, though, and if he is unable to beat Gutierrez, that may end his F1 career.