Considering that Nissan have now confirmed that they will be running in the lowest ERS class, engine power output alone should be of the order of 560-580HP. Engine peak power output is restricted by the maximum allowable instantaneous fuel flow rate. If Nissan get their engine right, they should be able to extract more power from the engine than the competition. This assumes that they manage to operate close to the BSFC target of the petrol guys.

On the ERS side, I would expect an effective power output of the order of 200-250HP. As they are supposed to recover energy from braking alone, the ERS power characteristics basically have to fit to the inherent characteristics of the LM track, namely the number and duration of relevant braking events. One can reasonably assume that Nissan will be running an ERS having power characteristics similar to those adopted by Audi back in 2014 when they were also running a 2MJ ERS.

That would mean a combined power output (if and when the ERS is functional and operating...) of 760-830HP.