It would be beneficial if RED users would post which filters/plug-ins work and which don’t, based on expericence. There are certainly enough demo and try-before-you-buy versions out so that you don’t have to spend any money finding out.
From my experience, Tiffen DFX 2 (demo)does not work with RED 4.x. I can’t speak for the original version of DFX.
Aurorix, Beserk, Damage (demo) and Delirium from Digieffects work with RED 4.x with the following caveats: Several of the individual Beserk filters do not work (they crash RED)and some of the interface features of Delirium don’t work, like the presets, but all of the filters work, as far as I can tell. I’ve have just a little experience with the demo of the newest version of Delirium and that seems to work as well.