I drive a S4 RX7 With a standard 13B Turbo, only mods are a straight through exhaust and a k and n filter panel in the stock air box, standard boost.
Best i could get out of the 55 litre tank was 260 kms, ive heard that with injection on rotors you can get a lot better economy with an aftermarket computer.
but yea, i don't reccomend as a daily, i drove mine for around 6 months as a daily when i first got it. the amount of extra fuel it used could have covered rego/ insurance ad fuel costs for a corolla.