is the rear spoiler on passenger cars just deceration or do they improve performance? For example the small spoilers on newer Ford Mustangs.

on most passenger cars, the "spoiler" is merely decorative. on some, it may have a minor aerodynamic effect, but factory spoilers that actually produce downforce are rare.

Well this one is easy. Spoilers were put on cars in the 60's because they has rear wheel drive muscle cars. The force on the back tires would cause the tires to rise up off the ground a little due to the horse power of the car. The spoiler was installed to keep the back tires on the ground giving the car more traction and makes it go faster. Putting a spoiler or aka wing on a front wheel car does nothing some think it looks better.

It depends. Some cars can really use them or badly need them.

On some cars, the spoiler enhances mpg at highway speeds.

The "wing" on the Audi TT is very critical since the car becomes real unstable at high speeds. There is a number of consumer cars out there which have a serious design flaw and need a spoiler or wing to keep them stable.