Just because carmakers don't publicize specifications of future models, doesn't mean that they're not obliged by law to register prototype vehicles created for development purposes if they intend to use them on public roads.
In some cases, depending on the country and local rules, they have to provide specific details. If we are to believe a report from the BMW loving crew over at the F30Posts, one of their members discovered a registration document for an M4 Coupe test car through a VIN check (there no mention on how and where).
In it, we see that this particular BMW M4 Coupe is fitted with a 3.0-liter engine codenamed S55 (it's a straight-six turbo) rated at 310kW, which equals 416bhp and 422PS. By comparison, the current M3's naturally-aspirated 4.0-liter V8 delivers a maximum output of 414hp (420PS).
Even if the registration document does prove to be legit, you should keep in mind these may be preliminary and not final output figures.
Story References & Renderings: F30Post
Hat tip to Kevin!