He's only 25 so I suspect that he will naturally improve as he reaches his peak years but I think it's unrealistic to expect (like I've read) Mourinho to improve him drastically because while they are both very similar in terms of their attitude and passion, we shouldn't forget that his current manager is Simeone, who just oozes passion. I doubt Costa would be playing this well without Simeone's management, so it will be interesting to see how he does fare away from him.