Greetings Perrycas and welcome aboard,
As an owner of diesel i30's I can't offer much advice on this problem, however things to check wold be feeling all wheels at the end of a long drive 30 mins, and see if the rims are excessively hot, could be a binding brake.
Remember it's an auto, so there will be losses due to transmission inefficiency.
Driving style will obviously have an effect. Avoid harsh accelerations & braking.
What were the road test figures that you have read about, how much difference are we talking about ?
I see you're in Sydney, so just how much stop start idling are you doing? Unfortunately in a petrol engine it makes a huge difference. My Commodore returns around 9.5 l/100 on the freeway but quite happily rises to 14.5 if I'm waiting at traffic lights. Naturally, I try to avoid that as much as possible.
Hope I have helped you in some way