Glad to see another XFE owner around here.
I have an '08 XFE and in my 28,000 miles have had a couple tanks pop over 50 mpg.
I'd like you to join the mpg team I formed (Team Chevy).
EcoModder Team Competition
I noticed the same things with mine and contantly climbing MPG as that car got broken in.
As long as you don't have a lot of stop and go driving and aren't forced to go 70 mph on the highway you should be seeing 40+ mpg before you know it.
Feel free to message me so we can share any tips, tricks, mods, etc....