The symlink should only influence the files that would have been in the game's install folder, not anything in your mydocs(and that is the temp location for downloaded patch stuff... assuming you didn't also symlink and move the my games/ffxiv yadda yadda/ folders). You will need enough space on your C: drive for the full patch, though. Your shifting 25GB is from a combination of temp, prefetch, pagefile, and possibly recycle bin files getting written or wiped, so you can't count on having it available. If you have any portable files at all, videos, general downloads, whatever, best use a USB stick to offload them at least temporarily.