The AT is a top heavy bike and a bigger tank will make it even more. If you can deal with it, is your choice, go for it.
But there are other means, like the side panels tanks. You can buy a side panel tank here... http://www.boano.com/special.asp?id=51&mod=no
which will help in keeping the weight low, while adding 17 liters of petrol.
Another radical move, will be to build a tank where the airbox/ or battery box resides, if possible. I've heard about this before but dont know if it is possible.