A postscript to @70MM13's post:
For Mac users!! I tried to format a custom 4Mb block size in exFAT using the terminal like this:
sudo newfs_exfat -b 4194304 -v NameThePartition /dev/disk3s3
It appeared to have done so, but I couldn't get it to mount under any circumstances. The card will work in the camera – but you will be unable to mount it to retrieve your files.
The closest I could get to a larger custom block size was 1Mb (1048576) (These numbers have to be powers of 2 apparently.) This will mount, but it caused problems in the camera and I started to get only about 15sec @ 3072p with or without card spanning/sd overclock on or off.
Not sure if it was related but I then kept getting card errors: 'Card1 cannot be accessed' after formatting custom block sizes from the Terminal. Reflashed the Firmware, finally erased formatted to exFAT from disk utility with the default [tiny] block size of 32768 (0.03Mb).
All appears to be ok again now. But bottom line may be that Mac really doesn't play well with exFAT/exFAT custom block sizes. I wish I'd left well alone!

PS If you format the cards in-camera I noticed (both my SD and CF cards being 64Gb) that the SD card gets exFAT, while the CF gets FAT32. Just noting…