Well, the vast bulk of information from the scans done on breeder cavaliers in the UK -- where breeders say about half have come up at some point with SM in their own discussions in the past -- would all be of intact dogs. And almost all the cavaliers in the 550+ sample used for one study on incidence would have been breeder MRIs of their intact breeding dogs, mostly from the UK and Nehterlands, not pets. So you can likely assume those figures from that study all primarily apply to intact animals, not spayed animals. You could email Clare Rusbridge on this.
In addition in the UK and Europe it is far less common to spay or neuter pets so I would wager the majority of all the cases of SM reported in the UK are in intact animals.
I thought the issue of growth plates had been responded to by the researcher doing the foetal tissue research who said that all indications are that the changes take place as early as during foetel development and that the brain seems to keep growing beyond what will fit the skull well before the age at which animals would be neutered. Maybe I am recalling an email or a question at one of the SM events but it did come up. Perhaps direct a question to the researcher, Imelda McGonnell.