The XS-Pro mount is slim, but with a front thread.
According to TDP, a standard B+W UV (F-Pro) does not cause additional vignetting, and it's cheaper...
+1 and confirmed.
I just tried a standard (i.e. not slim) 77mm B+W UV on my new 16-35 F/4L IS and it does not vignette, even at 16mm. I'd buy the standard one for best lens cap attachment.
This is the exact one I tried:
BWUVMC77 77mm UV Haze MRC 010M Filter
For me, that means not buying one at all, as I have at least one (maybe 2) surplus 77mm F-Pro MRC UV filters. One came from the 70-200mm f/2.8L IS II, which surprisingly for a telezoom lens does have additional vignetting with an F-Pro mount, so I replaced it with a 77mm XS-Pro.
Disclaimer: my feedback was from a quick F/8 shot against a white wall with no peripheral illumination enabled. I checked at 1x in the corners and saw no hard obstruction in the field of view at 16mm.
Non-pro question: I only thought filter rings were a threat to obstruct the field of view with an abrupt black corner. But can they also create a more gradual darkening like shooting a lens wide open?