Diffusers will always be less efficient. On some cooks I use them and others I don't. I started out using a 16" perforated pizza pan but now I use a 14" solid pizza pan with 1/2" holes drilled in it. I used it on a 15lb brisket smoke for Thanksgiving. I don't yet have a 3rd rack to lay the diffuser on and haven't made any type of holder to keep the diffuser a couple of inches off the charcoal basket so I didn't quite fill the charcoal basket so the diffuser would lay flat on the basket. I opted for a low and slow overnight smoke after 16 hours, I ran out of fuel and the brisket wasn't quite done. I added fuel and the brisket still turned out fantastic.
So what was the difference using a diffuser? The temperature is more consistent from the center of the drum to the outside. The end of the flat seemed to get more heat and cooked more evenly with the point.
OK Joe Vertical Smoker, UDS, Komado Acorn, Weber Kettle Ranch, Weber Kettle, Stok Kettle