Effect of raising rear of car?

Raising any part of the car will also raise center of gravity. You would ideally want to have front and back of the car at equal heights for serious road racing, all else being equal.

As far as aerodynamics are concerned, you ideally want to minimize total frontal area in relation to the "relative wind" to achieve the least coefficient of friction. The more body area that directly contacts the "relative wind," the more drag. This can be countered to a degree by helping the "boundary layer," that is the air moving along the body panels, to ride smoothly along the skin and not separate, which causes low pressure areas (i.e. drag). Raising the rear of the car will provide more down-force, but it is negligible until you hit high speed.

Chrysler did some major testing on car bodies for high speed NASCAR racing in the late 1960s. I have read their wind tunnel testing reports for the Charger Daytona and the Superbird. They found that raising the read end increased drag. The nose cones smoothed air flow and greatly helped the boundary layer. They also changed the rear glass to that of the Charger 500 to reduce the low pressure area behind the stock glass. The biggest secret was the wing, which obviously increased down-force on the rear tires, but the greatest asset, and which Chrysler kept a deep secret, was the tremendous lateral stability increase provided by the wing's vertical stabilizers.