SHOdded may be correct, a compressor gone or going. a leak would be preferable, but a consistent leak could put major strain on the compressor.
Sbouvia, is the hissing sound the only one you notice? what about a droning sound coming from the car when the AC is on?
Also, in the MKS we have an air recirculation button, do you have this in the SHO?
try pressing it and see if the AC runs any different.