According to my SA, the reason for the decreased HP in the X5 40i vs. the X3 (M)40i lies in the new Twin Power Turbo which is larger but runs more efficiently (and also with reduced capacity) compared to that of the X3.
Wonder what the X5 40i would put out at full capacity
What probably underlies all this is the increasing regulation (e.g. in Germany) regarding CO2 production, exhaust filtering (until now only for diesel engines) etc.