Just came up in the German i3 forum:
http://www.goingelectric.de/forum/bmw-i3-range-extender/staerkerer-rex-t4980.html
In the i3 price list of march 2014, the REx is described to have 25 kW at 4.300 rpm.
Later list has changed, now it is 28 kW at 5.000 rpm.
Looked at my own documents, my REx...