Do not go buying new O2 sensors and coils just to see if it cures the fault, get the fault codes read first! I had the same problem lately, as the car was under warranty the dealer checked the code (P0420) which means high signal on Bank 1, sensor 2 (inefficient catalyser), and replaced the sensor (£76). Twenty miles later the Motor Control System Failure light came on again, same code. So I spoke to an Alfa specialist who told me that with the JTS, 9 times out of 10 it is just a bad connection with the O2 sensor plug, and to clean them all as well as possible with electrical contact cleaner. So far the code has not re-appeared. Please try cleaning the connectors first before spending all your money on new sensors.
Another thing: you say there is no loss in performance? If you had a broken coil, you would DEFINITELY notice a change in performance.