Bottom line is "Yes" the spolier is effective for 80 lbs of downforce (or 80 lbs of reduced lifting force) at the rear of the vehicle.

As stated... if you turn off the spoiler and drive on the road (preferrably open, straight and no traffic) you will notice the rear of the vehicle becoming "loose" as you get nearer to 62mph. I would NOT recommend you go faster than this to experience this. If you go faster than 62mph, you do so at your own peril.

As others have written very well... the car's rear half is shaped like an upside-down wing and will create lift at speed. The laminar flow being disturbed disrupts this lift enough to keep the rear end very stable at high speeds.

Lastly... look at the amount of design and engineering that went into the mechanism and control system for the pop-up spoiler... you don't really think they spent all the time, money, wind tunnel testing and engineering effort for looks do you?