The Faster, Baby! DLC is a relatively short romp through the new territory of Sinclair Parish. Unlike the standard districts you won't get to flip any rackets or assign the place to an underboss. You also won't find any new collectibles here (though after the maingame, that may be something of a relief). What you do get are some new roads to tearup, some new missions, a meticulously detailed weed-growing minigame, and some unlockable new cars and a weapon. The DLC can be activated anytime after the first big sit down with your three underbosses, and the characters will all act as if that in the recent past. No one will mention the events of the endgame, even if you've already played through them, and even characters that died in the last act may still show up here.
When to Play
If possible, it's best to start this one early- the cars and weapon granted can be helpful in the main missions, and the weed-grow minigame require hours of realtime waiting that can be deadly dull if there's nothing else to do. The extra income also means more early on than it will during the endgame.
Wiring Sinclair Parish
Don't worry about it- there are three junction boxes, and the main story will take you to each of them in turn.