It is funny because when I drive the wife's X5 4.8, I get 14-15mpg within the city while she gets 13mpg: basically due to my driving in DS mode and shifting at 2000 rpm.
On the long trips I have consistently gotten 23mpg driving in DS mode again.
I think you should be able to get better mileage with X50i.

One thing though, it is not a fair comparison to compare someone's urban mpg with yours because the traffic situation could be worse in one case than the other.