I believe If you are seeing a hard corner vignette when you shift the lens, only in the top and lower corners first, you are seeing "the disc" without the disc you would see a more even edge and it would not be as hard, much more blurred as it's the edge of the sensor box you see.
I have used the 23, own the 28 (and know the disc is there), borrowed a 32 and it showed the hard edge at around 14mm, and demo'd the 35mm HR, which also has the hard edge. The disc I refer to creates a hard creates a hard rigid edge on the 28mm, 23 and 35mm. If you shift a Schneider 35mm or 28mm you won't get the hard corner vignetting. The 23, 28 and 35 HR's all show the disc sooner than than the 32mm since they have a smaller 70mm image circle instead of the 32mm's 90mm. It might be that Rodenstock read some users reviews complaining about the disc and removed it from later versions of the HR or HW series of lenses. I would personally love to have it taken out of my 28mm.
You can also read through this post from a review on the 28 Rodenstock and Schneider. http://forum.getdpi.com/forum/lens-accessory-reviews/34991-rodenstock-28mm-hr-vs-schneider-super-digitar-28xl.html
Guy discusses the issue of the disc very well and includes examples. Take a look at response number 5 where Guy shows the LCC's of the two lenses. On the Rodie 28mm you can see the disc cutting into the shot at 8mm of rise. You can also see the even worse issue of the penumbra of the image circle on the 28mm Rodie. The penumbra is is the white banding you see before the actual black of the disc. The penumbra will ruin a blue sky on a shift as you can't correct for it with a LCC and since it creates a lighter band before the dark of the disk, the correction of it can be very time consuming.
The physical CF is a user discretion issue. I tried the 28mm without it and found the center of the image too bright (due to the vignetting of the corners) The physical CF does even out the exposure very well, however I agree you pay for it with the 2.5 exposure stop difference. For me the noise in the corners was pretty harsh even at iso50.
The actual design of the physical CF's by Rodenstock will not cause any additional vignetting and or loss of image circle. You can shift to the "spec" of Rodenstock without seeing any vignetting. In fact you can add one additional 95mm filter to the 28mm's CF and still not see any vignetting. It's a considerable expense but does help out quite a bit in post. The 32mm goes from 86mm to 105mm with the physical CF installed.