hehe, you can't get exact figures because they do not exist - you won't notice a difference in speed from 100 members to 200 members, and probably not 700 members - it's all in flux. While your site may have 50% of its membership posting 3 posts/day, the next guy might have 20% posting 17 messages/day/each, with another 40% lurking, and another 10% only showing up in the early mornings to post 35 posts/member/day. My friend's site doesn't get many posts, but he'll have 500-600 or so online at once during most of the afternoon, even when he moved to a dedicated server and then to threads, his server continued to crash until the hosts finally configured it correctly.
You've been here for years, surely by now you understand that if your forum begins slowing down due to it getting larger with more traffic that you have to either upgrade your hardware/software or move on to a database solution - it's rather simple. And once you move to a database solution (doesn't matter which one), if it isn't configured well, it could run even slower than classic.
