There's an interesting piece in Auto Motor un Sport (the german Autocar) about engine performance this year.  James Allen has commented further and given his contacts in the paddocks seems to ratify the conclusions as being accurate.

More importantly it nicely sets up a great rock-paper-scissors season next year with teams either having power, or fuel efficiency, but probably not both.

The two most notable paragraphs are concerning how much time difference the engine performance actually makes:

"Most teams reached the conclusion, based on acoustic analysis and GPS, that the spread of engine power from the best to the worst engines was less than 2.5% this year. This means that, if the Mercedes is believed to have had 755hp, the least powerful engine was 18hp down, which is worth just under 3/10ths of a second per lap."

"The Renault won out this year on fuel efficiency, however, which was pretty important this year and will be much more important next year with no refuellling. Compared to the Mercedes, the Renault would go four laps longer on a full tank of fuel, which is worth about 3/10ths of a second per lap next year."

So although Mercedes has the best engine this year, it's likely to be even-stevens over a whole race distance next year.  Add in Cosworth with more power and even worse fuel consumption and it could get very interesting, especially if the new teams produce half decent chassis so they qualify up front.

I can't wait and there's still 5 months to go!