Any cross breed with cavalier in it will run equal risk of Syringomyelia as an untested pure cavalier.
You may have access to more up to date information, in which case please share! But from what I can find, it is believed to be genetic (although the mechanisms are not yet understood), is highly prevalent in Cavaliers, and has been reported also in Griffon, Miniature poodle, Yorkshire terrier, Maltese, Chihuahua, Bichon frise, Staffordshire terrier, Pug, Shih Tzu, Dachsund, Pincher, French bulldog, Jack Russel Terrier, Pekingese and Boston terriers.
So a cross with a breed not on that list would surely have a much smaller chance of developing the disease?
And a cross with one of the breeds where the condition occurs, but rarely, would still have a lower chance of the disease than a pure Cavalier?
And a young dog which tests clear is still not guaranteed to not develop the condition later in life, is it?