There is no clear percentage of what number end up with symptoms but there's lots of anecdotal evidence that the number is growing, proportionally, and that symptoms are becoming more severe at a younger age across the breed.
If 70% of *asymptomatic* cavaliers have SM by age 6-7+, according to a study of 555 dogs across four age groups (none of those seen to be showing symptoms by owners, but neurologists say many dogs owners think are symptomless actually show clear symptoms on clinical exam) you can extrapolate out that a significant number will eventually show symptoms. Breeders themselves who scan their dogs typically report about half have syrinxes already by breeding age or older. The number with symptoms will be high enough to make this disease a significant concern for any owner of a cavalier.
As I said, I have had five, three with *symptomatic* SM. If you have one dog with a lot of symptoms it becomes pretty easy to spot symptoms in other cavaliers, often dogs that owners think have no sympotms. My vets confirm the same thing -- once they knew what to be looking for, they have sent many cavaliers for MRIs that have come back positive.
Sadly I must disagree that the majority of dogs with Sm do really well after surgery or with medications. Studies show most do not, though many get by and a small number do really well. A fifth of dogs tend to worsen very quickly after surgery. Most dogs I know that have had surgery require ongoing medications to control symptoms, and most dogs on medications require endless adjustments to cope with increasing levels of discomfort/pain. I know several dogs with surgery that have declined significantly over time, including several that had the mesh surgery. In the one small comparison study done, about half of dogs on medications eventually are euthenised for pain that can no longer be adequately controlled.
Living with a more significantly symptomatic dog is an emotional rollercoaster, as others have noted. I feel very fortunate to have my most affected dog somehow cope with a wide and short syrinx with only moderate symptoms but his cocktail of meds have had to be increased and changed many times and even as he reaches 8 (diagnosed at 1) I still constantly wonder if I should have opted for surgery and watch him for deterioration. Deciding whether to do surgery is a whole separate, very emotional decision with many factors to consider.
I think most neurologists (and experienced owners of CM/SM dogs too) feel the number of dogs wit clinical symptoms is far, far higher than acknowledged because so many simply do not recognise or acknowledge visible symptoms and so many vets are uneducated about the condition, and themselves misdiagnose on average for two YEARS before the correct SM diagnosis is made, generally by a neurologist. There's growing evidence that CM, which affects nearly every single cavalier (researchers have found only about 6 cavaliers without CM) can cause significant pain inits own right. There's also plenty of evidence that the dogs simply learn to live with chronic pain. This means owners may feel a dog that has serious pain has none and is therefore without symptoms. The most common and debilitating form of pain the human CM/SM sufferers report is severe headaches -- something very hard to diagnose in dogs (though many of us with affected dogs do learn to see when they seem to have this type of pain and can add additional levels of pain relief).
It is a miserable disease -- considered the single most painful malady humans can have. The only way to even begin to address it, and hopefully save this breed, is to support health focused, testing breeders. If the poor breeders cannot make money off this breed they will either move to improve breeding practice or get out of a breed they do not deserve to be in.
I cannot stress enough that MVD is equally horrific and affects far MORE cavaliers at a significantly symptomatic level that truly impacts quality of life of both dog and owner. It was much more difficult and distressing for me to manage my older dog, who died recently from MVD years before she should have, and who was very symptomatic for many months, limiting my day to day activity and ability to go away -- than it has been to manage my SM dogs to date.
I consider MVD and SM to be equally devastating.