I use the pizza pan covered in foil sitting right on my basket as a diffuser almost all the time now. For Thanksgiving did a 17lb brined bird with a couple chunks of applewood on the UDS for the first time. Temps kept at 375-400 with all vents open and the valve maybe 2/3rds open. Put stock, apples, carrots, celery, onion and fresh sage in the pan the rack / turkey was sitting on. Also put same aromatics in the bird. About 2.5hrs in I checked the breast temp and it was at 160 already! I was expecting the pan below full of liquid / stuff to stretch the cooking time but it certainly didn't. Bird was done a full hour before I expected it to be.
So for me and whatever little differences my UDS has (it's pretty standard though I think), a diffuser doesn't seem to make temp variations etc. I think I understand why some like to have the drippings add a flavor dimension but when I went without a diffuser, I got flare up pretty damn quick but with that said, it was usually when I left the lid open too long so it was my fault
Anyway here are some before and after pics.