I've been running a 41 litre TT tank for quite a few years now. I find it perfect for travelling outback and anywhere else for that matter.
I usually end up with a 35 litres between stops but once last year I did 37.5 litres at which stage the fuel pump is just starting to hunt for fuel.
Generally speaking, you do notice the full tank, but one has to remember that it's after the bike has been running empty so you should feel the difference.
You have to rotate the fuel pump and also change a supplied metal wire, that holds the actual float.
I have seen and read about getting rid of the air reservoir at the top of the tank by drilling holes which allows one to hold 2 litres more fuel. Having seen what expanding fluid does to containers when it gets warm, I wouldn't suggest that it's a safe route.
Generally speaking I work to this standard, two up heavily loaded, sandy tracks 500 Klm's, normal tracks 600 Klm's, bitumen roads and hovering around 100 - 110 Km/h for touring, 650 Klm's. If I'm solo then add about 10% and if the bike is totally empty except for the rider add 15%.
I have actually done 770 Klm's on one tank. This was really an accident as the last town I was going to refuel at wasn't there, it was just an intersection of two roads, forcing me to travel sedately another 120 Klm's.
Mick.