Thie highway lt/100km, i can relate to, but it's a 5.4 V8. It will always use more juice than a 4 or 6.
In the city, getting 18-22 lt/100km requires some pretty heavy left foot. It is easy to get those readouts, but on average i'de say around 17-20.
But with an XR8 and that exhaust note, how can you help it?