With the D6 I haven’t noticed any difference, except for downloading a card to your computer. CFE is significantly faster.
Steve Perry reported some improvement in D6 burst depth with Sony CFe vs. XQD (171 vs. 129 frames or something like that) when shooting 14-bit NEFs at 14 fps but no difference with 12-bit NEFS or 12 fps (200 frames using either card), so one could argue the benefits in-camera are very minor. My experience so far is only with 32 GB and 64 GB Sony XQD cards and they work fine for my purposes, the buffer is very liberal.
However, with the D850, especially at 9 fps, it is possible to run into the buffer limit. This has happened to me a couple of times. CFexpress so far has not been reported to produce an improvement in D850 burst capabilities so it is up to future cameras to take advantage of the fastest CFexpress cards.