Lordy wrote:

After a bit of Googling it looks like they are snake oil (or at least "suck
it and see") as far as CPU cooling goes. Would they improve airflow /reduce
noise when used on 80mm case side vents? Or is pressure as important here
too?


Might have some marginal value on an intake fan (acting as a plenum) but
pretty much snake oil for an exhaust fan.

Yes, pressure is a factor and the air is going 'the wrong way' for it.
Axial fans sling the air radially, not 'forward', but a fan reducer is
trying to force the air in the opposite direction as it narrows down.