The world champions appear to be in fantastic shape heading into the final test, with the F1 W06 Hybrid barley missing a beat on its way to completing the most laps of any car in pre-season. Even more ominously for the opposition, the Silver Arrows have yet to unleash the full potential of their new car; Rosberg setting the second-fastest overall time on the final day of the last test on medium - and not supersoft - rubber. The big question is: will Mercedes show their hand this week or will they keep their powder dry for Melbourne?
After a slow start in Jerez where several power unit problems restricted them to a total of 735 km over four days, Red Bull had a much more positive outing in Barcelona, with improved reliability helping them complete more than double that distance (1,945 km). At the final test, the Milton Keynes team intend to focus more on performance, with the driveability of the Renault power unit a particular area Daniel Ricciardo is keen to focus on.
Much like Mercedes, there's a sense that Williams have yet to reveal the true speed of their new car, the FW37. Like the Silver Arrows they've shown near bullet-proof reliability and were able to dedicate a significant amount of time to pit stop practice at the last test - a clear sign of confidence. Both Valtteri Bottas and Felipe Massa have spoken positively about their new charge, and Barcelona could be the time they choose to lay down a marker for the new season.
After their headline-grabbing performances in Jerez - where they were fastest on three of the four days and set the two fastest overall times - Ferrari had a slightly more low-key test in Barcelona. The Scuderia did, however, manage to complete their entire programme without problem, and thus head into the final four days of pre-season in a positive frame of mind. Expect Sebastian Vettel and Kimi Raikkonen to do race simulation work, as this is an area that the Italian team have yet to dedicate significant time to.
McLaren enter the final test with their backs against the wall - not only have they completed the fewest laps of any team, but they'll also be without star signing Fernando Alonso who is still recuperating from his accident on the final day of the previous session. Kevin Magnussen will fill in for the Spaniard alongside Jenson Button, though he's unlikely to get much running unless McLaren and power unit supplier Honda can get on top of a niggling MGU-K issue which has put them well behind in their test programme. Rarely has a test been so important for the team.
After missing the first test completely, and then contesting the second with their 2014 car, Force India will finally have the 2015-spec VJM08 for the final test at Barcelona. That said, build time delays mean they will miss the first day of running, meaning they will have a maximum of just three days to shakedown their new machine before the season opener in Australia. Will that be enough?
Having racked up more than 3,400 kms over the first eight days of testing, Toro Rosso have met their main priority of giving rookies Max Verstappen and Carlos Sainz Junior as much seat time as possible before Australia. Add to that a good turn of pace - particularly on day three in Barcelona, when Verstappen was second overall and quickest outright in sector three - and the mood is understandably positive. The aim now, according to team principal Franz Tost, is to "come back even stronger" for the final four days, when a raft of new aero upgrades will also be introduced.
Eight days have been enough to suggest Lotus have risen from the depths of 2014, and that they can harbour far higher expectations for the forthcoming season. Improved reliability has given the team the biggest boost, while the fact Pastor Maldonado and Maldonado Grosjean went fastest on separate days provides evidence of the strong underlying pace of the E23 Hybrid. Promising signs then - or as Grosjean puts it, "it's always good to see your name at the top of the timing monitors, even if it is far too early to know what everyone else is doing."
After springing a surprise at Jerez, Sauber's Barcelona programme was more fragmented due to a variety of mechanical issues. "We cannot forget we are in our pre-season tests," was the verdict of the Swiss team's head of track engineering Giampaolo Dall'Ara, "where it is good to discover and fix these kind of issues." Given the slight setbacks, a clean four days of running will be imperative as the team look to switch focus to longer runs and race simulations in order to hit the ground running at Melbourne.