I'm not specifically familiar with this cars internals however my experiences tell me that most overheating issues involve heat exchange issues caused by either fouled radiators (water or air side) or coolant flow issues caused by thermostat failure. Flushing the system is a very good first step. When you do it, be sure to note the color/condition of the effluent. Brown is associated with corrosion which would be the primary culprit for most cooling system failures. Make sure you reload the rads with Ethylene Glycol (antifreeze) The thermal transfer rates are superior to straight water, it contains corrosion preventative, and is flat out required in environments were freezing could be an issue. The stuff is sweet tasting and deadly to small animals like dogs. May I suggest a pet safe type.
Be aware that other Ferrari models have issues with design piping where it becomes manditory to properly vent the piping to allow for proper flow. I have no idea if that is true on a 550. Someone else chim in please.
If flushing does not do the trick, inspect the rads for airflow restrictions, rad fans for proper operation, and thermostat (part number 8 below) for full cycle range and thermal operation. It should be easy enough to cycle with an ice cube and a lighter. (easy on the heat, don't burn it)
A thermostat is nothing more than a thermally operated valve the purpose of which is to limit coolant flow allowing the engine to reach operating temps faster. As the preset temp is attained (its Bi-Metallic and not adjustable), it is supposed to open thereby allowing coolant flow and maintaining the desired temp. Time and Corrosion is the enemy. Any restrictions in thermostat operating ranges cause coolant flow restrictions in flow that in turn cause overheating. They must operate freely.