400 cfm will be fine considering when you add your ducting it reduces the cfm anyways. also for future reference its 6 x 16 and 6 x 24.. some people may get confused the way you worded it but no big dealshould i go with the 550 cfm or 400 cfm 6 in 16 or 6 in 24? for a hurricane 6 inch 435 cfm fan i heard if the fans bigger than the filter cfm rating it sucks faster then it can filter. 35 cfm i don't think is a big difference but what if it dosnt suck enough