I did progressive springs in the front. Even if you don't have the big tank, I would suggest the upgrade for anyone with a GS800. I'm considering a Ralle-Moto damper as well.
I haven't done anything to the back to this point, but I will probably upgrade in the future.
The front is definitely a bit mushy from the factory. The rear shock is also pretty much standard BMW -- uninspiring. I have Race Tech parts at both ends (GS3 shock in the back, heavier linear rate springs up front). It is a whole different bike now. Still not a 'dirt bike' -- but it is a lot of fun on the B routes.
A steering damper makes a big difference on these bikes too. I'm really happy with the Scotts damper on mine.
As for the range . . . yea, it's a bit lacking from the factory. Running hard (keeping in mind that 'hard' on a near 500 pound dual sport is very different than 'hard' on a plated dirt bike), I get ~150 - 175 indicated miles when the light comes on (leaving me with ~1.1 gallons). That puts me in the neighborhood of 220 miles before I have to start pushing. I wish it held another gallon so I could leave the spare gas at home.