I think WP is not the issue here. It is probably your database, and server spec. 300k post is surely a lot, so you need a ton of space to save all that information.
There isn't a maximum of posts for WordPress. One thing though, it's better to have hundred awesome posts than 300K copied posts that won't get any traffic.. I've seen so many blogs having thousands of posts, but are as dead as everything. But ofcourse that's up to you
SonnyCooL (8 March, 2010)
So ofcourse if it works for you, go for it, but I would try to go for my personal way.. it seems to be better and many of us are doing it too
Aren't WP posts stored in a MySQL database? Maybe MySQL has its limits along with server resource limits.
Also, I know this is a generic response, but are Apache/PHP/MySQL up to date?