[Vincentiu]

I believe that, as it was partially said before, "is" in "if he is such a good doctor" means that the doctor is said to be a good one, regardless the time, of course, but that the speaker is rather skeptic. On the other hand, using "were", as in: "if he were a good doctor", changes the actual meaning in the respect that the skepticism of the speaker is greater; it is less likely of him to be a good doctor.
Yet, not only can we talk about the probability criterium, but also about the 'origin' of the information regarding the doctor's ability: the first type of sentence implies the fact that the speaker is using information that he only heard of ("they say that he is a good doctor"), whereas the second type should be more dependent on the speaker's subjectivity.
I must admit that I am not absolutely sure about what I have just written, but this the way I understand it.
As far as the alternatives are concerned, I chose the first because I think it fits better than the second one. I do not reject the latter option, but "is", in this context, almost necessarily means "should": "Given the fact that he is such a good doctor, at least this is what they say, we would expect from him to get that diagnosis right".