I would not recommend carrying gasoline in any manner other than in the OEM fuel system of the vehicle, as gasoline is one of the most dangerous hydrocarbon blends in our daily environment. Even with the stock system, care should be exercised to prevent accidents during vehicle fueling, expecially from static electicity discharge and spillage.
Regardless of grade, gasoline is extremely volatile, explosive and enery dense (not to mention carcinogenic).
When sealed in a closed container, temperature variations produce large pressure differentials; vented containers leak volatile vapors and, very often, liquid fuel. Think of not only yourself, your bike and others on the road, but what it could mean to have a presurized gasoline bomb parked in your garage.
Purpose-built and professionally installed auxillary fuel tanks are a less risky alternative, but still significantly increase the hazard of fire (or worse) and are subject to a variety of installation vagaries and deterioration over time. As non-design elements, though, they alter the weight and balance factors of the vehicle. Those of you who are pilots can appreciate impact of factor on vehicle dynamics.
I know that many people have run the risk of carrying additional gasoline without incident, buy why take the risk in the first place? Careful planning, fuel efficient riding practices and common sense are the wise alternatives until Suzuki decides to increase the fuel capacity at the factory.
If extreme range is the major factor separating you from happiness, keep a Concours (7.5 gallon fuel capacity) as a backup bike for those special situations.