I have used both the $33 plate and the actual Hunsaker fire basket in both the 18WSM and 22WSMs.
They both work fairly well to even out the temps in the cook chamber. By that I mean, I haven't found the need to rotate meat that is hanging in there (they all come out pretty even). I also haven't found much of a difference between the the plate and the basket in terms of functionality. The diffuser flavor is much more subtle than just hanging over coals (which I...and many others prefer).
My dad has a plate for his 18WSM and I have one for my 22WSM. They work fine as a diffuser and do seem to eliminate hot spots.