True - none of us know the actual targets & metrics used by Merc.
As outside observers we can see that their overall performance isn't much better (or perhaps worse) compared to last year. however it is a long season and they could get things sorted in the later half. Any engineer would be impressed to see that come together. The indicators of Bell, Costa & Willis's influence will be reliability and rigour behind the scenes, and that will show up later in the season when we see if they have stomach for a long tough season.
In terms of perception, Roseberg's win may have artificially raised expectations, peaked too early. Given one win in a season it would be better towards the end so they could point to improving performance over the year. Otherwise human perception sees a race winning team become a race losing team.
Cold statistics would see a singe race win as a moment of luck, being in the right place at the right time, the dice just happened to favour Merc that day. The biggest factor isn't the glamour of a win, but being trounced up by Lotus. That's worrying because it means Merc need to raise their game just to stand still, ie 4th best team. Merc will have hoped to be challenging for the big 3 but they see the team from Enstone leapfrog them to steal that honour.
Lets see how the rest of the season plays out, lets see if Lotus can keep their advantage over the whole season.