If you have enough tabs open in Safari, the overflow appear in a pull-down menu to the right of the always-visible tabs. The Safari UI changed a while back such that the pull-down only has the names of the tabs that aren't all visible in it. I preferred the old way - the pull down had the names of all the windows' tabs. (Why? The amount of visible text is usually not enough to figure out what the tab is, and the mouseover text takes too long to display.)
I've looked through the output of
defaults read com.apple.Safari but didn't see anything obvious.