As the other guys said, the temperature sounds normal to me. I get anywhere from 1/3 on the gauge to 2/3, depending on driving speed/outside temp etc.

Also as you've got a later XR8 you've got the 3.45 diff ratio, which will give you a slight disadvantage in economy to the standard 3.23. Maybe only 5-10% worse but obviously faster take off. Mines a 3.23 ratio and last time I filled up I got 542km from 61 litres, which is a lot better than what i expected - worked out to be about 11.3L/100km.