STS/HOSP gives absolutely no priority at all - it does not even exempt you from flow control. Most of the time to gain exemption from flow you have to fill in a form signed by a medical authority!
We have an operator who puts STS/HOSP on all it's flight plans, whether they are just positioning to pick up a patient or what. In my mind there is no such thing as a pilot declaring himself a CAT A flight. The flight categories are ATC cateogories that we allocate to an aircraft based on the information the pilot gives us, e.g. a pilot declares a MAYDAY - ATC afford him category A status.
IMHO the only way for a hospital flight to gain priority is to declare an emergency owing to the condition of the patient, in much the same way as any other flight with a sick passenger would declare a PAN. ATC would then afford the flight category A status. In fact, when i first started MATS 1 used to say just that!
Having said that, there are some pre-approved callsigns that give automatic priority - HEMS flights spring to mind.