I deleted a lot of them but still have 800 pages of pm's
Are you serious about this? That a lot of pms if you ask me, are they love letters or what? I believe you haven't had the time to even glance through 500 of them...
Try your best to give some if not all that little time to know why it was sent before deleting them.
But I am not sure if a subject title would help much.
I think a subject title would even be of great help to you than you can imagine.
Imagine all pms that comes to you have "titles" all on them, with that you don't need to even read further or even opening the message, just by reading only the title and you're done.