Yamaha Steinberg FW Driver Utility (3.) was the key. After checking all those - and literally every other old times computer voodoo stuff - combinations it is/was changing IEEE1394 Buffer Size from Small [Default] to Large in the end.
I had to do this in Windows 7 after upgrading From Cubase 7.5 to Cubase Pro 8. 8 was unusable with the identical hardware and settings I have used successfully in 7.5. Of course, my system latency increased by doing this, despite the claims of better performance in this version.