You can shift the 23mm and 28mm max about 6mm, and at 6mm you will start to see artifacts from the IC indictor that Rodenstock uses.
If you are pushing past 6mm, then you will hit a hard black vignette on the upper left and right corners of the shift and lower left and right corners of the shift. This is pure black and becomes increasing destructive to the image IMO as you move anywhere past 6mm of shift. Makes no matter if you are adding rise and or fall and shift. You still hit the edge of the IC by 6mm, and IMO 5mm.
You will also start to see a white band, (I used to know the optical name for this), which will show up before the black vignette. This is IMO not a problem on the parts of the image where you have non sky, but on a pure blue sky or similar solid it's effect can be very hard to remove the the LCC will not correct for it, at least in the past it would not.
Both the 23mm, 28mm and 35mm with the 70mm IC's will all show this to some extent. You have to get to the 32mm or 40mm with 90mm ICs to where you can get to 12mm of shift (Max for the XT).
For wides IMO shifting past 12mm is sadly no worth it, as both the 40mm and 32mm show considerable amounts of retrofocus distortion as you get past 12mm, (edge elongate and flatten), Very Very easy to see on any object with "known" dimensions (car, truck, Builidings etc), but not as easy to see in a landscape image (rocks, water etc.) but can be seen on tree trunks.
So for me the XT if the cost for entrance to the product was not so extravagant, I would consider it. But the issue is trying to sell NOW with the current market and world situation makes selling other lenses non XT or Cambo mounted not possible, as the loss is too extreme. I also am not a big fan of sending in a current lens for remount (from Arca to XT), just too many irons in the fire too many hands and shipping issues and no idea currently how long such a swap would take.
XT needs a realistic longer lens, as the 135mm at around 15K, (is just not very realistic and really not that long anyway on a MF camera). The 120mm Schneider is excellent but no longer made and the older longer Rodenstocks, 180mm 210mm currently have no plans to be mounted for the XT.
Best lenses for shift past 12mm ones that will hold up IMO are the 70mm Rodenstock, (currently available for XT I believe), the 90mm Rodenstock yellow\gold band (announced as now available a while ago) and of course the 135mm.
The 70mm, 90mm easily can get to 18mm of shift and hold up. Can't speak to the 135mm but I am sure at the cost, it will get there easily, albeit with a very narrow DOF.
Paul C