It's almost impossible to tell who will be the winner and loser here. All sorts of plausible reasons might swing the benefit one way or another. Curious that Mclaren were all for it though. At least that suggests they are more than happy with how their concept will behave without the additional flexing. Or they just have a clever design that outsmarts the deflection test.
I do think though, that it's benefits were quite overstated last season. Much like the rear wing and the alleged loss of performance at the end of last season that seemed to in fact only to be specific to conditions that were very well known to cause the car to struggle- cold through air temp or more than moderately wet conditions.