I eventually got the visa, but the process of obtaining the clearance from a nephrologist, added at least 2 months to the process, after the case office had been assigned. So I suggest you start this process now.
In the end, the doctor was very helpful and wrote an extensive and reassuring letter regarding my condition. But he did so only after requesting and getting the results of several lab tests (e.g. creatinine, ultrasounds, etc), on which he based his report. Note that I had not seen this doctor before, so he was not 'my doctor', hence the need for new tests.
In the end, given all the tests were OK, he could write a good report with 'migration specific' wording, such as "I see no reason for not granting this man his visa".
Also, he mentioned he does this "all the time", for various conditions, so you might ask beforehand if the doctor you go is familiar with writing reports for these sort of cases.