At different races the outcome could be better for Ferrari, supposedly the electrical issue on them meant both could go faster than they did, without likely expending much/any more fuel. Likewise, with the fuel limit at 100kg, at some races a thirsty engine may have to be slow to finish where at others it isn't a problem. Ultimately at those less thirsty races Merc can also put less fuel in their car but in general I'd say you have to go significantly slower to extend fuel efficiency than you can save by putting a few kg's less fuel into the car. If Merc really is noticeably more efficient that advantage could be much smaller at less fuel constricted races.
We could see Ferrari gain half a second a lap from electrical working, improvements in their brake by wire, and less fuel restriction see them much faster while arguably this could have been best case for Mercedes.
I don't think it should be underestimated what resources and time Merc have spent with the intention of being competitive this year though. Red Bull did the worst thing last year and over developed the car when everyone else gave up. Mclaren did the dumbest thing the year before in deciding on huge changes for 2013 and then 2014. Merc's focus on this car showed in the race pace performance of the past two years AND the performance in this years car, so really shouldn't be a surprise to anyone.
Unfortunately in this race it was too easy for Rosberg, he wasn't under pressure and was effectively cruising like Vettel has done way too many times. I think with the Merc drivers keeping each other honest and Williams/Mclarens starting from higher up the grid and not getting stuck behind, they should push each other enough to make it exciting. If Webber was half as good as Vettel and they actually put each other under pressure and caused mistakes in harder to control cars it would be exciting even if they were 20 seconds ahead of the next lot. It felt like we had more overtaking, more around the outside and inside of corners, more braking mistakes(though in a semi DRS way a lot of them have been induced by crappy brake by wire some teams have
). less Marbles and seemingly a lot smaller penalty for being off line. I think there were quite a few teams playing the "I'd prefer to finish than be fast" game this week. There were more mistakes and I think as more teams focus on pace than just finishing we'll see more pushing, more races where backing off for fuel isn't an issue we'll get even more mistakes which shakes up the order, causes faster cars to drop back and have to do more passing like Bottas.
The only things I'd really say I feel like I know for a fact, is that Merc is damned fast, Williams looks good in race pace and Mclaren look good also. Ferrari, Red bull, FI, TR, Lotus... not a clue about them really.