Supposedly Merc got comparitively worse throughout last year as they gave up on last years car to work on this years car, with more people, more staff and more money than they'd had to work on the 2010-2012 cars.
They also nabbed Lotus's aero guy IIRC, one of Merc's weaker spots(wasting a lot of time on a non working double drs, or not very effective) and putting more focus on what the car needs. LIkewise if Merc aren't miles out of it, they have no reason to stop development on this years car through the season. Last year had they improved drastically they would only have caught up with the top, too late in the season to gain any ground, it would have been money wasted. If you're up there in the top 5 every race then you have more to gain by developing the car further.
They've also got a significantly better driver, for me, best on the grid. Hamilton alone in last years car I sincerely believe would have done a lot better, qualified higher, better pace in race, etc, etc.
AS for teams being phased or not, what else would they say, Merc say they're slow and everyone else is fast, everyone else says they are slow and everyone aside from them is fast. I don't doubt most cars can go faster, but i don't doubt Hamilton can go faster as well. Currently the Merc and Hamilton together as a pair look the only team that has a shot at a sub 1:20.
A bit of a shame the "fast" drivers weren't all on the same day, Rosberg might be slower than Vettel/Button/Alonso tomorrow, and they go sub 1:20, but on a warmer day. Also possible Rosberg is fastest tomorrow and weather is worse and times are generally slower.
Force india looked to have not awful pace in the last couple weeks when they go their fastest, but Sutils longer stint times look pretty woeful at the moment, and if a semi accurate race sim that car appears to be losing huge time to tyre deg on heavy fuel, 5 seconds slower over 10 laps, second stint was somehow slower(maybe reusing tyres like they might have to in a race?) and 40 laps in the times are really weak.