1. Spoilers help fuel efficiency and high-speed acceleration by diffusing passing air, reducing turbulence. You're thinking of a wing.
2. Wings decrease acceleration on all vehicles, not just FWD. The added downforce increases pressure on the tires, which deforms the tire more, which increases the area in which the tire contacts the road, which increases friction. Also, wings create turbulence at high speeds, which sucks the car back. And the wing and downforce add mass to the vehicle.
3. Rear downforce, believe it or not, still adds stability, cornering ability, and braking ability at high speeds to FWD cars, too ... Remember how light FWD cars are in the rear ... They also play a very important role in RWD cars because it's important to keep the driving tires firmly planted while cornering to prevent spinning ...
