Have you seen the weight of those tourtanks? -5lb - over 2 kilo empty, and that weight is at the highest point on your bike (that's heavier than my tent and sleeping bag!). The first time my friend dropped his bike with one on, the brackets sheered off. Might be ok for cruising the highways in the USA, but for serious use, you would be better off with a plastic gallon can, you can buy them on your journey when you need them, and they weigh less than one pound. One other great advantege of the can - you can fill anyones bike up with it, not restricted to the bike it's on.
My insurance company would not cover my bike with a tourtank fitted without an engineer's report, and a reciept from an acreditted fitter.