Lowering a car has no direct affect on downforce unless the underside of your car is channeled to make best use of the air that travels under it.

What it does do is change the center of gravity on the car so that the forces on the tires become more parallel to the contact patch.

That said, camber kits for lowered cars are largely needed so that you don't get uneven wear characteristics on the tread.

The amount you should lower your car depends on your car, spring rates, tires, and road surface. For an average street car, 1.5"-1.75" is fine, but if you don't get a good set of shocks, and don't balance your spring rates accordingly, your handling could actually get worse.