The original note on your file was, as you say, purely an observation by your vet and not a diagnosis so shouldnt affect his insurance. Whether you already had him insured or took out the insurance after the observation by the vet, it should make no difference I wouldnt have thought.
Persians arent more prone to kidney disease than any other cat. PKD (which ultimately leads to kidney failure) has been a problem, not only persians, but many other pedigrees (and it is seen in our domestic moggies too). However, if the vet thought his kidneys were enlarged I would probably ask for a blood test to check his kidney function, then maybe do a scan, depending on the results. The scan will just show you whether or not he has cysts already, but it won't really tell you anything about his kidney function. To put you mind at rest though, the scan is not an invasive procedure at all if you did decide to have it done.