Slow tire leak- Tire shop or Maserati service?

If there is a reputable tire shop in your area, I don't see the harm in taking it there over the dealership. You'll probably pay less out the door and there's not much involved in a stem replacement. That being said there's always a risk associated with TPMS and they are costly to replace.

Is the leak from a puncture somewhere? If the factory tires are just leaking by itself for no reason, the standard factory warranty may cover the defective tire.

Just got back from tire shop. I had a puncture from a screw...Unfortunately it entered at an angle and the tire was not able to be successfully patched/plugged. They want to replace all four tires, as it is an AWD vehicle. Spoke to my Maserati dealer's Service Department and they say it's no problem replacing just one tire.