Just wanted to add my 2¢ worth. When I did my conversion, I couldn't justify the cost of an aux tank. My mileage on my 07, only dropped a few miles per gallon and I have heard exactly the same with the 18-23 models as well. When group riding with other 2 wheeled wings, we are all usually ready to fuel up right around the same time anyway. No one likes to run down to the point of the low fuel light on as if there was some kind of issue such as you could hit an unexpected detour or traffic getting shut down due to an accident, etc...
When alone and on a road trip, by the time I'm close to 1/4 tank, it's usually time to take a break anyway for a stretch, bathroom break, and refill of my favorite refreshment. At 1/4 tank, I have usually been traveling for 2-1/2 to 3 hours at regular highway speeds. Most areas, your at the most 5 to 10 miles from nearest gas station. Only once out west in the Texas panhandle did I get extremely close to running out and was down to less than 1/3rd of a gallon by the time I found an open gas station. Other than that 1 time, I have never wished I had the aux tank.