Bill, your experiment has me curious to learn how things work out for you after you have your box in place.
In the last 6 months I have built two different dust collection boxes. The first one was just a prototype to allow me to see just how much saw dust I could collect using downdraft collection along. The second one was the permanent one where I heeded the advise of Stick and others to make sure the motor had good clean air to breath. In both cases, I was and am absolutely thrilled with how much sawdust gets captured when dadoing and edge routing. I am never routing again without it.
When dadoing, I experience a slight burst of dust right when the cut is started, but once the bit is about 2x the diameter into the cut virtually nothing comes out on the table. It is all be sucked back through the dado slot and down into the box and out the dust collection port.
When edge routing, using my particular custom router fence (which has no dust port BTW), the dust from the edge route is sucked down through the insert plate as well. I attribute this to the 45mm cave behind the bit that traps, breathes and allows the dust to be sucked downward. Works so well, I do not see myself modifying its design to add the more traditional above table dust collection.
The added downdraft afforded by the extra holes in your plate may be all that is needed to improve upon what I perceive to be the weakest performing dust collection scenario - freehand edge routing (i.e. edge routing without the fence).
BTW, I am using the 7518 router, same as you.
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