The sDrive 18i will get a 6 Speed Auto, so no comparison to the ZF8 possible, maybe due to less gears it will be lighter which might help the overall package.
The xDrive 20d has a rated fuel consumption of 4.9l against 4.8l in the manual
But in general you are right that it should not be worse than the manual, also considering that with auto there is additional weight.
I didn't get the opportunity to drive one myself yet, however the ZF8 that I have is pretty good I must say.