Right up until you blow the welds on your intake.
We don't yet know how extensive it will be, but the following video is teasing a “standalone expansion” for Forza Horizon 2 which will allow you to play at least some of the cars from the Fast & Furious franchise. The teaser video features the Charger which is in at least half of the movies, as well as a blue and black GT-R, which can be seen in the trailer for the new movie. They're calling it a standalone, so we suppose you won't need the standard game to play, although that's just a guess.
There have been quite a few cars in these movies so far, so there could be an impressive selection. What a great way to live your life a quarter-mile at a time.