If a team wins a Fast Forward Pass, they are entitled to bypass all other tasks on the leg, and go directly to the pit stop.
If they have done some tasks, then they can skip all remaining tasks and go directly to the pit stop. (My recollection is that in some of the early seasons, a FF might not be provided at the last stop in country A, but would be included in the clue envelope at the first clue box in country B. So that is where this sentence comes from.)
In the case you ask about, the power of the fast forward overcomes the U-Turn, and the U-Turn would be treated as if the team had already passed that point in the leg when a later team attempts to U-Turn them.) So yes, the team doesn't need to complete any remaining tasks before the pit stop.
The teams keep the FF clue with then through the leg, the team can decide to go for a FF after reaching the detour, and IIRC, this did happen in some of the early seasons.