Air Filter Restriction?

I have about a 1300sq 2 story home. It has an outside 7 year old package unit, 4 ton cooling and 115K BTU gas heating. There is only 1 area where the return air vents are located, roughly in the middle of the house on first floor. Initially there was just one 20X16 return but years ago my parents were advised to add a second one to increase return air flow. The second one, also 20X16 sits just above the first one. Let me now say that everything works fine, both heat and cooling. My question involves the type of filters. I have been using the washable type. It has been mentioned that these are nothing more than monofilament fishing line and due a lousy job of filtering the air. There are times, when the window light is just right that I can see dust particles in the air. I have no objection to purchasing high efficiency filters BUT I have also read that the increased restriction caused by these filters can also place a higher load on the mechanical equipment leading to shortened life. Thought I would ask the experts before making any changes.

source https://hvac-talk.com/vbb/threads/2227019-Air-Filter-Restriction?goto=newpost