The McLaren was about .8 seconds off the pace of the Red Bull on average this year. We know for a fact the Renault engine is better, we know for a fact the Red Bull chassis is better. The McLaren Honda on average about 1.3 seconds off the pace of the Mercedes, we know for a fact the Mercedes engine in the Mercedes car is the best in the field, we also know for a fact the Mercedes chassis is better than the McLaren. What we also know for a fact is that the Ferrari engine is the closest to the Mercedes(but still not quite up to par), but we also know for a fact that their chassis is not as good.
Given what we know, we can assume that the McLaren chassis, as has historically been the case, is somewhere in the ballpark of the Ferrari chassis. I estimate then that the difference between a fully developed power unit and the one raced this year would be worth around another .5 seconds by itself, maybe as high as .7 with knock on effects, all it would do is put them on even footing with Ferrari.
Given that it's been obvious that McLaren didn't do much suspension development, and publicly stated that they were only focusing on things that could translate to next year by Spa, it's not a stretch to imagine that they are at least prepared for next year. I have a feeling that McLaren has been focusing for 2017 probably since the beginning of this year during winter testing. If Honda has had a separate program dedicated to 2017 from the beginning of the year, and they have gone on record to state that communication is very good, and that they have McLaren personnel in Sakura working on chassis integration, then it makes sense to think that they're prepared. Whether that will be enough remains to be seen, but I doubt they'll be fighting with mid field teams. I'm getting anxious thinking about it, we don't even have a release date.