350 - 400 on a tank is not bad.....other will say they get much more but then, I'll always say my VS is faster than yours.
Unless you actually find a fault in an item - e.g. O2 sensor ,TPS, injectors etc, it is pointless replacing them 'just in case they may be faulty'. Repco and Supercheap make a fortune that way.
I have a VS V8 ute, and would average a minimum 400k's to the tank. I would be checking plugs, fuel filter, leads, tyre pressure and ensure that any weight in the car (that is not essential) is removed (as listed above).
Also, not too sure how heavy your right foot is off the lights
How you drive has a lot to do with it.I can put my foot down in my VS ecotec and get woeful fuel economy,I can also get excellent economy by driving like grandpa.I mostly drive fairly quickly though,and Ive found that under these circumstances,the 98 octane fuel works out cheaper than 91 unleaded.Theres not much difference between the two fuels until you start to open it up a bit,especially around town.I go though 91 stuff heaps when I open it up,so I just use 98 stuff all the time as I use less of it.Another bonus is the car just generally runs better on 98.The other factor,and its already been mentioned is the state of the vehicle.Keep everything in good nick and serviced regularly,and it will be better away fuel than a car that never gets serviced.I find that some parts get missed or forgotten by by some home DIYer's,like the auto transmission filter and fluid, and the fuel filter which is tucked up under the car at the front of the fuel tank out of sight.A blocked fuel filter will cause low fuel pressure ,which can be cause of bad fuel economy.The oxygen sensors should also be replaced every 80,000 to 100,000km.They help a lot with fuel economy when they are working properly..