Looking into top speeds a little bit more, the below is a simple comparison between qualy speeds this year and last year.
I have taken Button from 2016 and Alonso from 2017 as in both years they tended to have the highest qualy speeds. For context I have also included Hamilton's speeds from 16 and 17.
Looking at Mercedes to start with, as expected their speeds are slightly lower this year with a maximum delta of 4.8kmh slower (S3). The only exception being the end of S2, which follows T12, here Mercedes have been able to gain speed since 2016 suggesting that they are taking T12 quicker than last year which more than makes up for the additional drag they are running this year.
Now looking at Mclaren they are again slightly slower than last year, they have a max delta of 6.6kmh slower at the speed trap, but only 2.4kmh slower at the end of S3.
Now we "know" (accepted rumour) that Mercedes have made a significant gain on their PU this year but we still see them posting slightly slower top speeds at Bahrain. With this in mind I would find it difficult to argue that Mclaren are down on power compared to last year, if they are then it is by an insignificant amount. This likely explains why they are using the 2017 PU rather than reverting to the 2016 spec.
The other area I haven't mentioned yet is S2 and the high speed T12 where in contrast to Mercedes, Mclaren are slower than they were last year.