I have a PS3 with a 320gb hdd, and i use to copy all games in ISO format to internall hdd (years ago), and has been a couple of times where i did filled the hdd "up to the border", lol
I dont remember how many free space i had... but it was the kind of thing i was not able to copy any other game and not able to make any big "gamedata" installation
And the PS3 was working normally, no lag, and no problems
But is needed to mention that i use to run that "restore filesystem" option from recovery menu very frequently (after every couple of crashes), so probably my hdd did not have much fragmentation