i have a 2006 euro Luxury and i get 550+ on every tank. This is suburban driving.
Only every use BP Ultimate 98.
only Mods that would effect fuel consumption are K&N short ram intake
toda headers
cat back exhaust
18inch riims. Pressure 40 front 38 rear

i have platinum plugs also and clean my air filter every 5,000kms.

i drove to narooma once from melbourne and had a passenger and a fully loaded car and it weighed a ton and i got there and drove around for 4 days on 1 tank. got 849kms from the tank.

i also found that using caltex fuel i get 50kms less - consistently.

i recommend going to BP ultimate 98 for a few tanks, change your plugs (platinum), and clean or replace air filter or get a high flow k&n filter or equivalent and reset your tacho and try again. can almost promise better fuel econ!