By the way, let me elaborate on my original question:
I am wondering how much the difference in performance will be, of having 7 databases instead of one with seven sets of wp tables. Thoughts, optimization tips?
One can have one single database with seven sets of wp tables in two ways: first is going multisite, second is using seven different worpdress installs, ALL pointing to the same database, but each using a different table prefix.
Silkalns' answer refers to the first case only, doesn't it? I mean, maybe the answer is the same EVEN in the second case, but not necessarily. Or not?