Reduce the load before chasing a rating
Filters create resistance to airflow even when clean, and that resistance generally increases as they collect particles. The HVAC blower must operate against the resistance of the entire duct system.
A severely loaded filter can reduce airflow enough to affect cooling-coil temperature or furnace operating temperature. Simply installing the highest available MERV filter is also not automatically appropriate for every system.
Good filtration balances particle capture with the airflow capability of the equipment and return system.