Thanks a lot Bubo for the insightful discussion. I still believe that ultimate coaching in any sport is mental and that part is universal. It's about understanding specifically the process in an individual's mind that leads to optimal performance and reproduce it at his own command. But this is too big a topic and I don't expect that everyone will agree.
Since you mentioned tennis grips, let me ask you for your opinion on the western grip (for the tenth time in these forums? I hope people don't ban me from asking the question again). My 10yr old son uses a western grip and claims that this is HIS grip. His private coach is fine with the grip. He does go to group lessons and camps, however, and at these places he is constantly reminded that he needs to change his grip to semi western if he were to fully realize his potential (most think he has potential). Even coaches at the Evert Academy in Florida tell him that (he went there for a week). With support from some expert opinions in this forum (Oscar Wegner, Coach, etc.) and seeing what Nadal and Djokwich is doing, I tell my son to ignore the naysayers. But still, it makes me uncomfortable when a lot pros say with conviction and, I believe, out of care, that he needs to change. Your opinion will be highly appreciated.