Originally Posted by sonoronos
The spoiler acts as a drag to airflow, but does so in exchange for downforce. There is no free lunch. The more downforce you make, the more drag is necessary to create it. The efficiency of our rear spoiler is unknown. This is known as a lift-to-drag ratio.
This is not nessecarily true, I'm at work and dont' have time to go into detail...but via looking around on the net you can find ways to improve downforce without increasing drag. Let me get home and I'll elaborate.