It's fairly easy to kill O2 sensors. It does depend on the sensor but if you get oil or grease on some and use a cleaner that isn't approved, dead sensor. Coolant will also kill an O2 sensor. If these are dead you need to replace them. They'll have an adverse affect on your performance and gas mileage. The ECU uses info from the sensor to calculate the proper amount of Fuel to inject into the cylinder. If it gets a dead reading the ECU will go into a SAFE mode. This means dumping more fuel than is necessary into the combustion chambers. Extra fuel cools the combustion process, it also isn't as efficient as all the fuel doesn't get burned during combustion. Causing a drop in performance and in mileage. If the air to fuel mixture were to go in the other direction you wind up either getting KNOCK (Predetonation) or hot spots that will burn a hole in either your piston, ring, or cylinder wall. KNOCK kills engines. So, better to be too rich than too lean.
Is it safe to drive your car like it is? Yes, your mechanic was correct. Should you continue? No. I believe the Thermo couples check the temp on the Cats so those no longer apply. It may be that your O2 sensors are fine but arenít being read properly by the ECU. Either way you need to get that fixed. Most O2 sensors (Bosh) are about $100-$200 each.
Congrats on the Tubi! Enjoy! But, you'll enjoy it more with an engine that runs the way the Ferrari engineers intended. That means getting the O2 sensors worked out.