I think they`ll have a tough decision between top speed vs efficient DF, bearing in mind they`ll run behind an RB car in the first part of the race (at least) and most likely with the help of the DRS ...
But I agree with you that tyre deg will have here a bigger influence rather than top speed or race pace and if I were Merc I would be focusing entirely on race pace having a 2 stop strategy, due to 2 factors: one is the high chance of an SC phase and the second reason is the fact that running in free air could unleash the true race pace they have, something they couldn`t do it in France ...
And speaking about a 2 stop strategy, what do you think it`s the best choice: S-M-M, S-H-M or even an S-H-S one?