I'm really not sure how serious of a problem the 5 stops of vignette would be in real world use. In his review of the rf 50mm f/1.2L, Bryan says that lens has 4 stops "peripheral shading." I don't know how to measure it in Lightroom or PS CC, but what I'm seeing in my f/1.2 photos doesn't seem to need so much correction...
Five stops certainly sounds bad. We laugh at people who complain about not being able to fix images that are five stops underexposed...
At least with the 50mm at f.1.2, most of my shots are composed in ways which are not too badly affected by the vignette. My ef 16-35mm f/4 has very little vignette, so, I'm left trying to add vignette in LR to see how it affects wide angle images shot with wide apertures, but how do I translate the Lens Corrections slider to "stops"?