96-98 RON, 300rwkw GT3076 with WMI kit blah blah, I went to the GC and back [round trip of from memory 1700km] with mainly highway driving but also had a day in Sydney and a few drives around the GC and I averaged just under 9.8l/100km.
But your motor is a lazy straight 6 until you boost it. At 110km/hr it is just in the brink of boosting and hard drinking - good actually, I know when I speed because the turbo starts whistling at me!!!!
And I kept a notebook for it, just so I could check, haven't really done it since - my burning curiosity was sated.
OP your fuel figures are crap, get a proper tune.