They surpassed the previous record of 263-5, set by Royal Challengers Bangalore in 2013
Dries Mertens scored from the penalty spot with eight minutes remaining after Andy Robertson tripped Jose Callejon inside the box.
Napoli substitute Fernando Llorente added a second in stoppage time to put the result beyond doubt.
In the other Group E game, Salzburg made an impressive start with a 6-2 victory at home to Genk.