peoman,
Just unloaded an '00 400EXC off the trailer yesterday. We rode at Rampart Range (approx 7800 ft) and Divide (9000-9500 feet).
The bike ran great with a 160 main and the needle clip 2 positions from the top. The needle seemed to make the biggest difference. The bike started out with a 172 in it and the clip in the 4th slot from the top and barely ran. Even with the 172 and the needle dropped it ran OK. The needle up though with the 172, it barely ran. Had about a 2K wide power band that started at 5k and ended at 7k.
A plug reading with the 160 and dropped needle still showed it as rich, so I'd take a few smaller pilots along. Drop it 2 full sizes as a good starting place, run the 160, drop the needle to the second slot, and you should be able to ride the booger. Oh, run the fuel screw 1.75 turns out and carry a tip to play with it with you.
This should remove any worries. Do the above and you'll be able to ride it no problem.
Mileage? The stock '00 tank is small, like 2.5 gallons. We did 60 one day miles but had to take about 1.2 gallons out of my XR. Still were not on reserve though when we got back to the van. This was with the 172 though, so I'd guess it was getting about 20 MPG that day. With a moderate throttle hand and the leaner jets I'd be planning around 22-23 mpg. My XR250 was getting about 40 in comparison.