I'm going to try and attach some images. I'll try to describe the setup. On top of the air handler is a "Y" where two 16" flex ducts are connected. The first split runs a short distance to a ductboard plenum(not sure if that is the right word or not, but it is duct board where all the flex duct feeds to each vent connect). This part of the split supplies air to 50 or 60 percent of my house. The 16" ducts in the picture are the ones that run across my unfinished bonus room and to the far side of the house. This supplies air to the remaining 40 or 50% of the house, and splits into a similar plenum as the one closer to the air handler.
I have a 4" aprilaire filter which I have updated to the most recent slide in filter type. The unit tends to perform well, although on super hot or super cold days, it does struggle to maintain or change the temperature. My electric bills have always been reasonable (I average about $250/month in North Florida in an all electric house with rates about .12/kwh, with a 2850 sqf house). The only room where we ever have a problem is my daughters room on the far side of the house with 3 exposed walls.
I've never had a blower door test done.