Just a couple words of warning... the subframe on WR's (and I'm sure YZF's) is super weak and known to break when loaded, I wouldn't carry much more than a full tool bag on the rear fender. With 2.5 gallons of gas weighing 16 pounds and miles of bumpy roads, there is a very good chance you could break the subframe and that would leave you stuck where you break.
Also, a WR is good for about 35 MPG... depending on how your YZF is geared, I wouldn't expect much better MPG's. With this kind of ride, you could jump up a tooth on the counter sprocket to reduce RPM's (better for the engine) and fuel consumption, but don't expect big change in MPG from these bikes.
I actually have a 6 gallon Acerbis tank in the garage for my WR, I would like to do some big mile, all-day rides on my WR but I'm a bit concerned about the engines low oil capacity. I think just over a quart of oil will break down pretty quick with lots of speed and heat.