I don't have a Keg GVP, but I do have a Keg-H and a Keg-F. Testing these to, side by side, yielded interesting results.
Air Temp in the garage was 50°F.
No breezes in the garage.
Water temperature was close to 33°F.
Water in my garage boils at 197°F
Fuel was Sunnyside Denatured Alcohol (~90% ethanol)
Once a boil was reached, the pot and cone were lifted, the stove was snuffed, and then immediately weighed.
I believe fuel weights are accurate to 0.2 grams, even with delays after fueling, weighing, lighting, snuffing etc.
The actual boil times are with 5 seconds - Light #1, Light #2, set #2 Cone, set #1 Cone, start the timer.
I used Reflectix cozys as lids. Lids make a Huge difference. If you are not using one, you should be.
The first round was for 2 cups of water:
The Heine took 11:35 for a rolling boil
The Fosters took 11:10 for a rolling boil
The Heine used 11.6 grams of alcohol.
The Fosters used 12.2 grams of alcohol
The second round was for 3 cups of water:
The Heine took 15:00 to boil
The Fosters took 13:20 to boil
The Heine used 16.8 grams of alcohol
The Fosters used 17.3 grams of alcohol.
In the photo below, note the level of the water -
It was a dicey deal lifting the pots, and the only thing between your fingers and boiling water is some very thin very conductive aluminum.
The photo below shows another use of a cozy -
This improves efficiency, but has a tendency to degrade the edge of the cozy. I don't mind, as cozys are easy and cheap to make.
My conclusion is that any differences in efficiency are small, and easily overwhelmed by wind and subtle variations in stove height/pot height.
Note that in this particular combination of components the Fosters boiled faster, but used more fuel. Perhaps my Fosters band has slipped, lowering the pot, superheating the alcohol, etc., etc.
In this test, the additional cup of water (50% more) was above/outside of the cone, yet the boil required less than a 50% increase in fuel.
Last, the risk of burns when dealing with 3 cups of hot water, in a "24 oz." can, is very real.