Hey chevy,
I don't know if this will help you or not, but, Acerbis sells a couple of different bolt on fuel tanks that would definitely fill the 3 litre gap that you currently have. My friends kid used to use the number plate tank on his KX80 a few years back so that he could ride the distances the big boys did and it worked great. The only draw back is that you have to ditch your light if you have one on your bike. The rear tank looks kinda dorky, but, you'd be able to keep your headlight. Only problem with that one is that it could make getting on and off in a tight/quick situation a bit difficult. Not to mention that it could give you a rather nasty shot in the back side. But, it's still cool that Acerbis offers these up as I could see where that turtle tank on the back would be great for some adventure touring in the wide open spaces out here in the west.
If Husaberg doesn't step up to the plate in the next couple of months and get clarke going on the bigger tank, I'm definitely going to be getting one of those acerbis tanks, probably the number plate tank as the places where I would be using it require me to get on and off my bike quickly sometimes, and it's super easy to put it on or take it off.
The place where I have in mind to use such a tank is in the high sierra's (8400' msl, with a density altitude of about 12,500') where the miles covered are not great, but the hours are high. Typically we'll be gone 5hours + on a ride and I'll still have some fuel left but not much. The longest rides in this area are about 50, but, you're going to be out 7 hours +, it's all first, second, and sometimes third stuff. Lot's of fun.
I do agree with Taffy that with proper jetting and engine set up you can definitely increase your mileage. But, just to give an exampe of the differences in useage in various terrain. In the desert, trail riding at a brisk pace without too much technical stuff I can easily get 70+ miles on my stock tank. Throw in some technical sections with a lot of sand and that number drops to 60, this all at 3400' msl. Now this past summer I was riding in the mountains of south central utah, starting out at 6400' msl, and riding up to about 8500 to 9000'. It was mostly jeep roads hard pack, I was riding easy, enjoying the scenery and making a point to short shift and roll on easy. I went 78.8 miles on 1.5 gallons ! But, throw in an agressive race pace and those numbers go out the window.
Hope this helps,