no use going from a 1.3 to a V6 and cry when you have to fuel it up..... naturally it will use more fuel, have a larger capacity tank, but i bet it's better to drive than the 1.3! When i had the Omega it ran better on E10 than it did on normal 91 ULP or 95 PULP. The SS has always been run on 98. The wife put E10 in it once, i thought something was wrong with it... it went from an average of about 12/100 to almost 14/100.
At the moment it has a tune, CAI, under-drive, extractors and zorst and it gets about 11.3/100 doing highway and traffic. Last few days i have been driving it around town and not been overly kind to it and it's hovering around 16/100
Fuel type and driving style makes a huge difference to your economy. Asking others can only ever be a very rough guide.