I have ADD and I am a pre-med student. If diagnosed, your diagnoses is confidential. The only way anyone might could find out is if you had accomodations or something, I am not sure on that though. I do know that if you have accomodations on the MCAT (like extra time), your score is flagged-meaning that schools know that you did not take the test under regular conditions.
I don't think that it would be a disadvantage to be diagnosed. I was diagnosed this year with ADD (inattentive type). I didn't immediately get acomodations for college. This is the end of my 4th semester and until now I didn't feel as if I needed any help or accomodations...but now I am looking into it. None of my classes before had time limits on tests, except for one class this semester, and that gave me some trouble. I then realized "what if I get into a more difficult class that has timed tests?", I would be in deep crap then. So I asked my disability support services at my college about ADD and accomodations for it, they gave me info about it and am in the process of looking into it and everything right now. I have already been diagnosed by my family doctor and am on medicine for it, but that isnt enough for the school; they want a diagnoses and a few tests run by someone with more expertise about ADD and learning disabilities.
My suggestion is to talk to the disability support services at your college. They can give you some info, like whom your problem needs to be diagnosed by, and the tests that are required (usually IQ, and an achievement test, maybe another one that tests attention) Some have a list of doctors or psychologists that offer this (mine gave me the name and number of one that would do the tests at a good price since my insurance doesnt cover the tests).
It definately wont be a disadvantage to talk to the disability services, or even being diagnosed. And I dont think it would be a disadvantage to have accomodations if they are needed. Its better to have them know about accomodations and do better in class or on the MCAT, than it is to have no accomodations and have poor grades and MCAT scores.
So, it would probably be more of an advantage that a disadvantage to be diagnosed.