On Youtube, the Forensic Detailing channel (guy from the UK) did a 30 minute review of the XFE7-15, reviewing not just the polisher itself, but also did a comparison between it, the Rupes and a DAS 6 Pro Plus, very carefully showing the pros can cons of all the polishers. Just look up "Forensic Detailing" using the Youtube search bar.
As far as I know, there is NO backing plate that allows the use of 5.5" pads for the XFE7-15, yet. I wpuld think that it will eventually be brought out (just like the XC 3401 didn`t have an official Flex backing plate for 5.5" pads for a while after it was announced). From what I recall, you can put a backing plate for 5.5" pads on both the Rupes Bigfoot 21mm. and Griot`s Garage 21mm. polishers, but their manufacturers don`t recommend it. The Griot`s Garage 15mm and Rupes Bigfoot 15mm come with backing plates for 5.5" pads, but the manufacturers don`t recommend the use of a backing plate for 6.5" pads.
No idea if there even is a KB washer mod for the Flex XFE7-15. I haven`t heard that it is needed, either. The only reason that the KB mod is needed is that the backing plate on those polishers flexes when they are being used hard. The washer mod gives a little bit more clearance between the back of the pad and the gear head housing (I personally think that that is a design defect, in terms of adequate clearance). The XFE7-15 seems to use a different backing plate attachment system (center bolt, plus 3 more out more towards the edge), so the plate rigidity may be higher, eliminating the need for a washer mod.