Approximate kilometres by each engine manufacturer during pre-season
Mercedes - 9618 km.
Ferrari - 4747 km.
Renault - 3816 km.
The overall miles done by each type of engine is pretty interesting. While the Ferrari is seemingly more reliable in terms of testing miles done and data able to be gathered by the engine manufacturers Ferrari due to low number of cars using it didn't get that much more running in total than Renault. So in terms of finding bugs, looking at data, maybe tweaking for power, efficiency and the like Ferrari weren't too far ahead of Renault thanks in the most part, to Caterham, though in terms of how useful the data is, Ferrari cars were going faster and for mostly longer stints.
Merc both due to the reliability of most of their runners and seemingly the engine itself got double the amount of running Ferrari did. That could be a pretty crucial difference come the start of the season and the engine freeze. Williams switching to Merc engines this year is probably turning into the best decision they've made in years.
Rosberg has said his fast lap was done in full qualifying setup, trim and fuel levels. Only about a second off last year's pole lap but on different tyres. The supersofts this year I'm sure I heard someone say they were like 3/4's of a second faster than the soft. Without using the same tyres as last year though I don't know if these tyres are faster or slower. They supposedly degrade less but that might make them faster due to being able to abuse them more or slower simply as they are less soft and give less grip.
I usually relate the idea that better wearing tyres should mean less grip but that doesn't have to be true, in any case with very little tweaking and first qualifying run being within a second of last year's pole not on the fastest tyre is interesting for sure.
We saw people saying on the live feeds(without know the actual times) that Merc/Mclaren were doing longer stints in the 1:39-1:41's at various points, which are pretty similar to the stints Vettel did in the race last year.