Small disclaimer: disregarding track specifics, and only having Miami as a sample.
I was wondering about this current stand-off between a works team and A-status customer team. While Mercedes being a works team gave them a head start, a big upgrade package has led to Mclaren bringing the fight to them while having the same engine, the difference makers are the chassis and aero. Surely Mercedes will bring updates themselves, but I do think Mclaren has a slightly superior in-season development curve. Supplying engines to Alpine and Williams isn't really a concern while Mclaren is an S-rank threat in that department.
Will they hold each other back? A better PU is obviously Mercedes prerogative, but it will also benefit Mclaren when they're already close to each other. They also have the same inter team dynamic at some level. A WDC and a promising talent who wants parity despite this, and a veteran languished in a midcedes car for a few years and a promising young talent who's leading the WDC. Neither Toto nor Stella can afford to give either of their drivers preferential treatment now. But Ferrari (up for debate) and especially Red Bull definitely can.
Even more so since Red Bull and Ferrari don't have a Mclaren problem, they're also PU suppliers but let's be real Vcarb and Haas will never be a threat in the true sense. And the drivers of these said teams, and especially Max, have nothing to lose and everything to gain. Of course all dependent on specific tracks and development, and surely reliability.
It's not 2014 anymore where giving a OP PU to a backmarker team doesn't make difference. And sure lower midfield teams can also improve, but Mclaren is already here.
So what do you guys think?