i also have a 2009 4.8L and averages 20L/100km and with a 145L long range tank averages 600kms, I do not have a sub tank "yet". Would better economy be nice, Yes but it helps if you need to put the boot in and get out of the way in an emergency
very thristy motor but a lot of power behind it. Make sure you look after the motor as a new 1 is approx $13,500 to replace (just found that part out)