There is no question a regular HDD slows down as if fills up:
http://macperformanceguide.com/Storage-WhyYouNeedMoreThanYouNeed.html
Normally I'd think the performance of a mostly-full Fusion Drive should be dominated by the HDD portion. However in this one test Mac Performance Guide did, performance stayed pretty flat up to the end (barring a big drop at the beginning):
http://macperformanceguide.com/iMac-Fusion-fill-volume.html
However that was only one type of I/O -- apparently sequential. It cannot be fully indicative of overall FD performance across a range of free space. I have done many tests on my 3TB FD, and at 40% free space the QuickBench 20-100MB test still achieves about 670 MB/sec read, 310 MB/sec write, almost the same perf. as 93% free space. So in that case there was no big perf. drop as seen in the MPG test.
I may have posted before that FD slows down the same as a HDD when it fills up; if so that may not be correct. I'll do further testing on this and post my results.
However 800GB free on a 3TB FD is only about 25%, so just from a management standpoint it's time to start freeing stuff up. Otherwise you might run out of space at an inopportune time. Araxis Folder Size Explorer is a good tool to examine space consumption:
https://itunes.apple.com/us/app/araxis-folder-size-explorer/id510309552?mt=12