Real queation: would putting fans in series like this perform (significantly) better?

They would likely perform worse. If ("if", ha) those fans are not in perfect sync, they're going to obstruct airflow. Also, consider that each fan introduces both audible noise and perturbations in the airflow which in turn, also will cause noise.

Incidentally, I have (almost) that very same case, an Antec P182. Mine has a super-quiet PSU fan, and a ginormous heat pipe cpu cooler (HR-1 if you're curious) with one fan ... and that is plenty.

But keep in mind that the sound of the cooling will be irrelevant once that user starts typing, as their mechanical keyboard will produce enough sound to not only drown out the fans, but also to shatter their eardrums

You say that as if that’s a bad thing

MX Green Switches 4 life