It seems that the $shout_limit only affects how many shouts are kept in the DB. If you want to display more of the history of the past shouts, you would need to increase both the $shout_limit, to keep more and a constant in the Java script file shoutbox.js, to display more.
This may have changed in later versions. I'm still using 7.4.2.

Line: if (x < (last_shoutid - 30) )