I've also got a merv-8 air filter on the intake side, which is surely cutting off some airflow too. It sits outside 24/7 on the side of my house, and that air filter needs to be changed every week because it gets so filthy. I've been thinking about taking it off and seeing how it does, I'm just worried it's going to get covered in dirt and pollen every couple of weeks.

Do you keep yours outside? How do you manage dust and pollen?