Here is a
BMW official press release that mentions the overboost rating too (see tech specs and horsepower/torque numbers).
I notice this document is for the Canadian market. Does that mean US models didn't have this feature enabled?
[EDIT: I just went to BMW USA and BMW Canada's website and the technical specs for the xDrive 28i make no mention of the overboost ratings, the US site lists 240 hp and 260 lb-ft, and the Canada site lists 241 and 258, at least for the 2015 models. If you check bmw.com, it actually does show an overboost rating in the tech specs for the Z4 and 1M!]