The Crossfire's spoiler is functional. It's 80 lbs @ 80 mph functional, if Chrysler is to be believed. I'm sure our rear diffusers have something to do with it too.
I believe Indy and Formula 1 cars use wings, not spoilers. Second, front wings are required on F1 cars because the rules prevent the use of underbody aerodynamics (raised footboxes are required, etc.)
You assume, incorrectly, that pushing down on the back of a car leads to steering problems. Pushing down on the rear of the car with 80 lbs applies very little lifting force to the front of the car because of the lever arm. Imagine the car as a see-saw pivoting around the rear axle. Our rear spoiler is less than a foot from the rear axle yet the front wheels (and about 90% of the car's weight) are a little less than 8 feet away. Basic physics will tell you that the lifting force on the front wheels will be a maximum of 10 lbs, and when you take into account that 10 lbs of lifting force acts on more than 2500 lbs, you hopefully see that the steering is, for all practical purposes, unaffected.
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.
Yes, our top speed would be reduced due to the increased drag. A small price to pay for that rock solid feeling you get in our cars at unmentionable speeds, in my opinion.
No, it will not affect steering. The DC Engineers did a great job making sure that the existing steering system took the rear downforce into account.
My name is sonoronos and I love engineering.