I have 300rwkw and I get 9.8l/100km...…..and I know I get this because I drove to the GC and back. So 2400km, up the Hume then the Pacific with a bit of messing around first in Penrith then in Parramatta before continuing to the GC.
I drove around the GC a bit going to Wally World's etc, then when I came home I went through to Lismore then up to Tenterfield and back through the west of NSW [Tamworth, Dubbo, Forbes, Young etc].
So not a lot of traffic [but still some], and mainly highway cruising 100 to 120km/h. Climate control was on, it was warm but not stinking hot. I reckon if you can't do that with a well set up mildly tuned Gtst then you need to do some navel gazing.
I do have WMI....not sure if this makes a difference though cos I only used 2.5 litres for the whole trip.
Biggest single distance was something like 519km between fills....but that's from memory, I didn't log it. It was over 500 guaranteed.