I had a '06 PW manual that got from 30 to 39. I just refilled my '06 TCM auto after a run to Portland Oregon from Eugene for Christmas. The TCM has 1700 miles on it...29.60... I am dishapointed so far...
In the winter when it's below freezing all the time I get about 285-300 miles out of a tank. In the summer I usually get 350+ miles out of a tank.
You will get better mileage than a stock xB if you do exhaust and an intake. Most cars actually lose a little mileage when you do those mods, but the xB is underpowered so when you do those mods you actually see an improvement in mileage because the engine isn't working as hard.
So far I'm averaging about 31mpg per the odo. Per tank, now that's a different question entirely. I don't like to let my tank get too dry, so that's usually about 275 miles (per the odo). Calibrated, it's about 5% more.