My 2 cents on this.. take it for what its worth.
If you looking to just add some capacity, the Sampson passenger seat tank will do that. It will give you 3.9 gallons, and in some peoples opinions will look better than your typical rectangular fuel cell. However, it will NOT fit with a Givi topcase. You will have to relocate the topcase back just a tad to clear the fuel cell.
If you are looking for maximum fuel capacity based on the 2009 IBR rules, then the Sampson tail dragger or the BLM (Top Gun) fuel cell is the way to go. Both of these will bring to much closer to the max fuel allowed per the IBR rules.
If you are looking to add capacity, have some skills, and want to save money, go find yourself a 4 gallon JAZ fuel cell and mount it where the passenger seat goes. You'll probably spend half the money as the other premade solutions, and you will gain considerable distance.
Keep in mind the 11.5 gallon max is ONLY for the Iron Butt RALLY not for any other rallies, or Iron Butt Association rides. If you are not entering in the Iron Butt Rally then you do NOT need to worry about this limit. If you are entering any other rallies, you should email those rally masters and ask what their requirements are.