Based upon what I've seen so far while driving the car in Germany, I think the sDrive X1 after break in will get around 32 mpg highway which isn't bad when driven around 70 mph in light traffic.
Overall, on a stay in Germany to pick up the car, I got 25+ mpg and that driving was perhaps 70% highway with some really slow city driving thrown in.. But on the on Autobahn, I must have averaged 80 mph and I pushed it to very close or over 100 mph a handful of times. In the US, I won't be driving those speeds so the mpg will be higher based upon what I've seen with the instantaneous mpg readout.
I do think the 34 mpg figure on the highway will be possible but that will probably require very light traffic and driving 65 mph which most people don't do