The plastic filter is there to give longevity to the spin-on filter, ithas a nice 10Micron element in it that catches gook. and it is visual to know when it is dirty. the spinon filter is more than adequate to keep your fuel clean. why slow down your fuel flow more with a third filter? the plastic screen flows fuel fast enough to keep up with the IP and doesn't reduce the pressure or flow much. the Spin-on has HUGE capacity so when smaller particles are captured, it still flows well.
if I were going to add an additional filter... and I have been thinking about it. I would add a large filter with a glass bowl and water seperator. I have a few from my old days of tractors and Mid West winters with #2 fuel. I have no idea if elements can still be purchased for those old filters though...
John