Do not forget:
BMW i < BMW < BMW M
The BMW i8 is a model of the sub-brand BMW i, and is not a supercar competitor, it is a new creation.
The supercar competitor is going to be the Z8, built by the sub-brand BMW M, and only available with M engine.
We can see 8 is the flagship number and ther are "going" to be 3 of them:
BMW i flagship: BMW i8: It is as we already know the Vision Efficient Dynamics, a sort of coupé with four places, fast, futuristic, ecologic.
BMW flagship: BMW 8 Series: This is the production version of the BMW CS Concept, a 4 door coupé, very luxurious and comfortalbe but fast with its BMW V12 engine.
BMW M flagship: BMW Z8: Even if ther is no M badge, this car is an M car, a lightweight fast coupé-convertible with only 2 seats, and probably a specially developped V10, from the M5 F10. It may be faster than any supercar out there, but will not be a Porsche 911 GT3 RS. For the 911 GT3 RS there is the M3 CSL F32 coming.