Stew and I were wondering how much of an advantage having a fresh engine would really give in modern rev limited F1. James Allen reckons this:

Because Alonso retired in Japan and took a fresh engine for China, his unit will be on its second race and thus about 10bhp down on Lewis’s by the time the races comes around.

That is quite significant on a track like this where horsepower is critical for the long uphill pit straight.