The guage will show full for ages (up to 70 miles).
Then it'll start to fall, but at a slightly slower rate than on the std tank (due to the Adv tank's extra width).
The warning light will come on at approx 10% higher mileage than it would have done on the std tank, but you will still have around 10l of fuel left in the tank (ie: approx 100miles to empty).
NB: If you're riding around the IOM at "Island speeds", the whole lot can go in 165 miles...
I picked up a second nand ADV tank and put the standard internals in and it worked fine but the gauge only worked as a standard 1150 - at least I never run out of fuel.
I believe that the only difference is the length and shape of the metal arm on the float.
Hope this helps.