Ok a little late here but nonetheless...
I have crossed the country a few times, been to Alaska and back, ridden Death Valley, and I have to say...
Within reason you don't need a gigantic tank! I have an 11 Gallon Ascerbis on my Airhead GS. My stock tank on my R1100GS worked just fine as long as you pay attention to your range and don't go crazy on the throttle.
Fact is, those *few* trips when you actually NEED that extra 2 gallons? A small gas can strapped to the bike (well) is plenty.
I end up riding around with 60lbs of fuel when I don't really need it!
but it looks cool, now don't it?
However, if I was riding an XR600 or something with a 2 gallon stock tank, then YES upgrade to larger for sure.
300 mile range is HUGE. Oh, and NON of your friends will have that much range, so...