Might wanna pop out your injectors and send them out to be cleaned and flow tested… eurodrive does 
As for me… I can get 480-500km a tank with (91) or 550km with (94) both are city driving with the odd expressway run. Car is running @ 9psi and un-tuned…
I’ve been thinking about it and wondering if I can get the milage any higher… I’ve already got a fresh oil change + filter, + fuel filter and air filter. O2 sensor is the stocker from japan and I’ve put 30,000km on it so I’ve wondered if getting a new O2 would add some milage… Only other thing I could think of was once I install my wideband/SAFC2 by leaning it out a bit that would help increase milage as you’d use less fuel…
One thing I would like to add thou is Turbo size… if you have a small ass turbo which is spooling at low RPMS, you’ll use more fuel up since the Turbo is always spooling… once I changed to a bigger turbo, the turbo wouldnt spool under 3000rpm and I instantly noticed my fuel milage go up…