My VS SII S Pack 5sp Manual V6 Sedan which I sold a few weeks ago consistenly had awesome fuel economy. I always do all of my own servicing and this helps with economy. The only mod to the car was a genuine cold air intake over the radiator.
I used to sit on 110km/h and I used to always get economy figures of 8lts/100kms. This works out to be 12.5kms per litre. I used to drive my VS normally and not drive it hard as my 10 sec VL in the garage is the car that gets the hard driving.
With every fuel up at the servo, I would reset trip meter. This then allows me to work out the economy figures in all of my cars with every trip to the servo. A good trick to see if your car is running well in similar conditions.
Servicing and tyre pressure is a key to great economy. I kept 36psi in front and rear, the oil was changed every 5000kms along with air cleaner element as I get trade from holden and it costs $7 or thereabouts. Put new plugs in every 2 years or less ($6.50 trade from my holden dealer for iridiums), change fuel filter, put some new leads on etc etc.
If all the normal servicing is kept up to date, then great economy in the 8-9lts per 100kms should be easy.
Oh, and I always only fill up with BP Ultimate 98 as fuel isn't the best anyway, so for a small extra cost, go for the best. It has cleaners to keep fuel system healthy and it is better on turbo cars etc. All my cars whether its a little runabout, daily driving VZ or my VLT Berlina see Ultimate 98 only.
PS Oh and if you stay on a constant 90-100km/h limit, fuel economy should be better than on 110km/h