After looking on Mcalren's performance relative to Williams, i believe that their power unit is less maximised than the williams package. Mercedes may have done this at the start of the season knowing full well that Mclaren will be switching to Honda.
I really believe that the ERS is more advanced in the Williams and Mercedes car.
In fact it would be silly of Mclaren to try and benchmark the honda engine to the Mercedes PU, as they would be selling themselves short. It's the long term partners like williams who are enjoying the fruits of mercedes labour.
Mclaren simply got a 6 for 9 from Merc when they declared that they will be switching to Honda.