Depends how hard the Merc's were pushing, they have the hardest pushed engine on the grid and with Ferrari moving to new ICE's that will boost their comparative BHP. Are Merc worried about reliability, would they go slower than they would like in FP2 and just extrapolate the lap times, they have software and teams of strategists/numbers guys for that kind of thing. But could that effect how hard they'd push in this race. THey might take a new engine after FP3, or even qualifying, in other words using this engine up as much as possible and saving the next engine a few sessions. Who knows, we'll see how it plays out.
This year could be strange, Ferrari have opened a second engine, meaning even if they haven't taken all the new parts they are locked in spec now so if they had token upgrades for any part they only get them on engines they haven't used when the upgrades are made.
Would a 3rd/4th here but opening out a new engine with 50bhp more next race at Barcelona and having that 50bhp more for 3 engines over the season be better for them than taking a 2nd engine here, winning but having 4-5 races 50bhp down on where they could be? That is pretty much where Ferrari are, new engine here but by taking it IF they had 50bhp update ready for Barcelona, have they now lost that extra power for multiple races?
Strategy on reliability due to the in season changes has changed how people will use their engines completely compared to last year.