Just as a data point, if I drive my 4.8i X5 in "D", I get a little over 18 mpg in complete highway driving. If I shift myself, I get 20-21 mpg. Admittedly, I shift BEFORE it gets to 2000 rpms. Yes, it somewhat defeats the purpose of getting the V8, but I am liking the fact that I am actually "beating" the advertized mpg. I usually drive 75-80mph on the highway.
Just thought I'd share my mpg experiences.