Hi,
Alan that is a neat plenum and it is all tapered down to the air exit point. I did a thread on my saw, because of this thread. I think that the main advantages of mine are that it is simpler to make and it is rugged. I think the main advantages of yours is that the walls of the plenum are sloped towards the air exit port.
In practice I am pretty sure that yours empties the plenum almost completely. Mine does not. There are a couple cups full of sawdust that remain in the plenum. The sawdust fills in the cracks and rounds the corners. The great majority of my plenum stays empty.
Your back plate allows for tilting the blade with the plate in position. My back plate needs to be removed for bevel cuts. This leaves my back wide open; it still removes the great majority of the dust. So it boils down that I prefer my simpler plenum and I prefer your more versatile back plate. I am going to make a new back for mine as soon as Glenn and I get my shop's North Wall skinned and French Cleated (I doubt if that is a word).
My back plate is held in position with two rectangles of Velcro. So it is quick and easy to remove (but that is just one more darned thing to do). Your back plate lets in more air than mine. That could easily be a better thing, mine may be too restrictive. I will know when I make one like yours for my saw.
Thanks for the info.
Enjoy,
JimB
p.s. I forgot, my thread is "Craftsman Inexpensive prep for dust collection." It was dumb idea but I made the new thread because I had a bunch of pics. I ended up using only 7 of them.