We don`t know yet, but 400HP is only wright if the weight off the car is gonna be low. Now IMO that will not be the case, so the HP wil be over 400.
With a 4.0 liter, a 100 HP per liter would be a shame for the best engine builder in the world, even the E46 M3 rated at 105 HP/L.
Conclusion: With all due
........................IMO 400HP rumours are BS