To be fair, 3rd place doesn't mean 3rd quickest. Ricciardo was in 3rd, as a consequence of Ferrari having a troubled weekend and Verstappen having a PU error. Given how the strategies worked out and Force India was right on the tail of Ricciardo with both cars, who knows - a McLaren with a Mercedes engine might just have been in a better position on such a power circuit. That's about it though, not more or less. If they had an engine somewhat competitive, I would imagine them battling for 3rd with RedBull - best case scenario (and RedBull as a result of their own engine is a second off the pace, but that is probably bound to change soon). Assuming Ferrari and Mercedes finish most races, that's still 5th/6th, and if Renault sorts their engine, that will just bring RedBull closer to the front.
It's all rather moot though for the reasons I explained above: PUs are NOT interchangeable without any cost. Change the PU and you are bound to change many other things about the car too. So it's not necessarily given that McLaren could run the same aero platform with a Mercedes engine. First of all - the weight is somewhat bound to the PU and as has been posted in the other topic (either the Mercedes PU or the Mercedes team topic) the overweight issues of the Mercedes is also connected to the PU.
