A high MERV or HEPA rating on an ambient filter doesn't really have that much effect. There is no well defined "clean side"; the clean air immediately mixes back in with the dirty air.
Filter efficiency affects how long it takes to clean the air not how clean it ultimately gets. A filter with 85% efficiency takes 18% longer than a 100% efficient filter to clean to a given level. Of course you do need a somewhat respectable efficiency, 10% taking 8.5 times longer than 85%.