Pretty much all helmets are tested to the same standards. Which means all helmets that pass testing provide AT LEAST a set amount of minimum protection.
Data on how much more protection than required that a helmet supplies is scarce.
There used to be a persistent rumor that cheap helmets actually provided MORE protection, since they generally were heavier and with less vents and cutouts.
If there ever was any truth to that, I don’t know.
But it’s easy to see where the idea came from.
Air helmets kinda-sorta already exist, look up ”Hovding”.
Whether they overall provide more protection has been somewhat debated.
The Hovding does better in some impact geometries and poorer in other.
Planning to land on a flat surface, the Hovding can do a great job.
Expecting to end your trajectory against a corner or an angle/ridge, better use a rigid helmet.