Wild Weasel,
It is the Civil Aviation Medical Board (CAMB) and
not SIA that decides whether or not you are FIT for a Class 1 medical. If you are really worried about your medical condition, consult an aviation physician or get a Class 1 medical privately before you apply for SIA. If you can pass the medical, well and good. But if you can't pass the medical, at least you can seek treatment and get into shape (unless the condition you have is a permanent bar from holding a license). Like someone mentioned earlier in this thread, it's better to apply to SIA when you are in a tip top condition, rather than to pass all the interviews only to have complications during the medical clearance stage.
For self education on medical fitnes requirements, you can read Part II of the Fourteenth Schedule of
CAAS ANO (7 Dec 2005 version).