I know there are quite a few members here with sites that meet the criteria. It's just a matter of if they are willing to send me a clone of their site. Unfortunately the one that is having the problem can't send me a clone.
How about this:
Create a test board -> create a script to open 500K accounts from the ACP (usernames are just numbers 1 until 500.000), same password.
-> once you have your users, create a script to make each user post 10 messages in a single thread (500.000 x 10 = 5.000.000)
Enjoy your Big Test Board