The reason people say wings are effective on RWD cars only relates to the small number of cars that have enough power to spin the tires at any speed, like top fuel dragsters, CART cars and F1 cars.
If you think about it, wings should be MORE effective on FWD cars in a race applications. Since you don't have the throttle to balance a car in a corner, FWD cars need to be tuned to oversteer more in order to handle well. Typically, to get a car to handle well in low speed corners, it will end up being loose in high speed corners. Adding a wing will add downforce to the rear of the car and theoretically balance the handling inhigh speed corners.