What's positive for me is the planned engine upgrade for Spain (if true) which is not that far off. Of course we don't know Mercedes' development plans but Renault 's are apparently not til summer. Moving forward with the engine is going to be the biggest chunk of development potential for them so positive that they feel they can make further improvements so soon after already taking a major step.
Williams themselves also think that vettel's lap didn't show Ferrari's full potential, and I also feel there's more obvious tightening up work possible on the rear than on some other cars, which speaks of good potential in the car.