You want run 5 different Forums in 5 different directories, and all forums use the data of one DB?
Ist that right?
Then install one Forum with DB, load up your Forums to the dirs with the same config.php. Dont install them.
Now log with yur account in and change the Base URL in ADMIN-->Options-->E-Mail.
Thats it.
But you can only log in in one, you can't logged in in all forums at the same time.
But why 5 Forums and one DB, in all forums is than the same stuff???
And the Performance, i dont know, I think thats the wrong way. I think its better to use for each forum a own DB.