my car has full 3" turbo back exhaust 3" high flow cat, front mount intercooler, pod with cold air intake and grounded boost control solonoid running 10psi.
i get 420 - 450kms per tank on 98 octane city driving with the odd squirt here and there.....and that is without the fuel light coming on, i never let my fuel get that low.
high way driving, and city driving, well over 550kms.
like others hav said, change ur o2 sensor, plugs, and maybe try running 9psi to stop ur ecu running rich
goodluck !