Honestly, basic science years dont reflect on ability. Anyone who gets through medical school can be a 'good' doctor. Half of any doctor's success is first of all based on patient interaction. Even if theyre asssholes, they can do well so long as they can at least put up a good face in front of people.
As far as knowledge goes, thats what clinical years, internship, residency, and fellowship are all for. If you see similar cases presented to you year after year after year, youre going to be able to recognize what is going on.
We have a well known professor who often says that the doctors that get the best residencies are the B/C students (from first two years). His assumption is that they will work that much harder in clinicals and get great LORs as compared to the students at the top of the class. Also, those with a ton of clinical experience tend to not be at the top of pre-clinical education--but will obviously shine during clinical education.