The heatsink stacks have been optimised according the performance of the fans too, which have an improved pressure to airflow curve. This has allowed Noctua to reduce the fin clearance from 1.9mm ...

The Noctua fan used by Prusa is designed to provide airflow through a case, not force air through a heatsink. While the Bear extruder improves the Noctua efficiency, it is still not powerful enough.