I would say ball park about 3 or 4 stops is the worst case scenario with the greater movements. When I say greater I am talking about 17 left or right for example. Obviously the lesser the movement the lesser the light fall off. The digital centre filter than neutralizes the light fall off in photoshop. It pisses me off that you have to do it this way because
1 You have to take notes of every new lens movement and aperture. Not that, that's big a deal. I use notes on my iphone and email it to myself so I have a digital recored.
2 After processing the images in phocus and getting rid of the colour cast using the calibration shots you than have to
3 Use the plugin in Photoshop to get rid of the fall of.
4 If it's an architectural commission as opposed to a landscape shoot, I have to forward a lot of proofs to my client. I process the RAW files first as JPG fast previews. The images are processed practically instantly. I than have to use the plugin to remove the light fall off in photoshop. Many of my images as you saw are panoramics. I than have to do quick editing on the images to make them look more impressive when viewed at about 800 pixels. Once the client picks the images I than have to do the whole thing again using the high res TIF Files. You might think this is doubling the work but it takes about 20 seconds to process a tiff fill from phocus. I normally have minimum 3 exposures per image and many of my images are panoramics. All the editing in photoshop takes a lot more time with the large files from a medium format camera. Also because of things been so bad in the construction industry here in Ireland, clients are quite often severely restricted on the number of images thy may purchase, irrespective of the quality of the shots. They might only buy 6-10 images out of 25 - 30 views. They still want all the views.
5 with larger lens movements you sometimes have issues with noise. This can be removed using for example Nik Define for example, but it's still more time wasted.
Bottom line is my workflow is gone to hell with this lens, but I'm telling you now there is nothing else like it on the planet. The shots capable with it are just unlike anything else out there due to the movements possible with a lens of this focal length. Plus it's sharp as hell.
It's a love hate relationship but unquestionably it's the best purchase I have made since getting my medium format camera/view camera system