What do you think happens to the performance of the P1/918 when their respective battery packs become deplete and there is no charge left to power the electric motor??
A P1/918 on track will be able to re-generate 'some' of the energy used up through brake-regen, but battery based brake regen only recaptures at best 30% of the kinetic energy, it's not very efficient. Just had a look the P1 has a 4.7 kWh battery, which is a very small amount of charge. Unless McLaern have found a way to defy the laws of physics, the battery will only supply the electric motor with power for a very short period of time. Ofcourse clever electronics can be used to use the motor very sparingly to help 'boost' the 700bhp+ ICE motor at certain rev point etc...BUT essentially it's a very very quick petrol powered hyper car.
Which is exactly what the GT-R will be. It won't haveanything remotely like the systems found in F1.
The P1's motor is used for torque fill in the range before the turbos spool, which is what makes it so monstrous. The 918 uses one motor on the rear to assist the V8, and one on the front to drive the front wheels to give 4WD. Either way would work for the GT-R, although I'd expect it to be using perhaps two motors on the rear wheels to give at least some resemblance to the LMP1 car for the sake of advertising.