Nick, try to see if there's any oily residue around the front of the A/C compressor. Our cars use a Harrison (now Delphi) V5 compressor which is known for front seal failures, and leakage from between the case halves. Rebuilt compressors are available from several sources.
Another problem area with the 550 are the hoses leading to & from the condenser. Ferrari in their wisdom routed these hoses under
the frame rails, so other than the undercover, they're the lowest point in the system and if/when you bottom out, it's likely the A/C hoses that are cushioning the blow of the bottoming
. They wear through eventually and require to be replaced, and the sharper techs will also reroute them to a less exposed area.
Lastly, the schrader valves in the gauge ports are known leakers, especially on the high side. Fortunately Nissan schrader valves fit just fine, so don't get talked into a new hose for $600 if it's only a schrader valve that's leaking.
Our compressors are the first generation of variable displacement technology, so they run all the time, whenever the A/C is on. They do not cycle like most compressors, but adjust their output automagically to the needs of the system. Bottom line, if the A/C is "supposed" to be on, have a look at the A/C compressor and see if the clutch is engaged. If it's not, there's something wrong with the system, and it's liable to be a leak.