This is either an option for hot races or a compromise they had to do in order to unlock extra performance from their Power Unit. Either way, it was a change that was definitely made with performance in mind, which may or may not be adopted in the future. Just because they're using it for testing doesn't mean it's definitive.
I'm just wondering what some people here expect from McLaren, something like "We need bigger sidepods to extract more power from the PU, but let's not do it because then the car won't be so pretty", or maybe "Let's not test this higher airflow spec because it may upset some fans which won't find the car that pretty this way"?