800 rewrites in one htaccess file sounds like a lot to me-- especially if apache would have to check every single page access in that folder against 800 rewrite rules. You might consider a RewriteMap directive that uses a text file. The trick in that case would be for the new URLs to have some kind of easily identifiable flag which prevents them from being tested against the lookup step.
Another possibility is to put the redirect in the old file. If the old file is HTML, a meta redirect would work. If the old file is PHP, you can use [man]header[/man] and send a 301 redirect. If all of your files are PHP and they all include one centralized init file (which is a really good idea, generally speaking) then you might be able to use some kind of simple logic to accomplish the redirect in one place rather than editing 800 files.
