Caporegime
Sometimes the fastest cars in testing (top 1 or 2) are running on fumes trying to make a statement (pull in sponsors) - although the top teams rarely engage in this. However the plum slowest team of tests rarely (if ever) start winning races because they have been sandbagging. Being plum slowest usually means you have built a lemon (although you can sugar coat that as you like - as most teams do!).
Yup, there's usually one or two midrange teams that overperform in testing and don't deliver come raceday; and the exact order and margins are hard to discern but I don't ever recall a complete switch around.
Having said that: this is still early in testing. I wouldn't be too surprised to see Mercedes move forward as testing progresses.