The specs for the BMW i8 in the BMW USA website have changed recently and it reveals that the car goes 0-60mph in 4.2 seconds only. Pretty impressive numbers!
The horsepower though was lowered from the earlier estimates of 362hp to 357hp. This actually makes more sense since the power peaks of both engines overlap in such a way that the combined maximum is 357hp/ Though the individual electric motor peak output is 131hp and the Turbo-3 is 231hp.
Source: BMW USA