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w the roof the air is forced to follow the roofline which is a longer distance than the air travelling under the car (a curve vs a straight line). conservation of mass and all that says that the air traveling over the roof has to get to the back of the car the same time as the air under the car. since it is a longer distance over the top it has to travel faster (red vs green) to get to the back at the same time as the underneath air. the faster air is less dense, which creates a pressure differential btw top and bottom of the car - this is lift. the purpose of the rear spoiler is not downforce (otherwise it would be a wing) but rather to 'spoil' the airflow over the top to kill the lifting effect (the spoiler creates a pocket of turbulent air which causes the airflow to delaminate from the roof further forward than it would otherwise, forcing it to travel straight back instead of along the car - shortening the path).
w/o the roof then the curved path is gone and the spoiler not as necessary. replaced by a big pocket of turbulent air - less lift, but it takes work to make air turbulent, so increased air resistance.
the soln is to keep the roof (less air resistance) but kill airflow under the car to reduce lifting effect - front lip, side skirts. add a rear diffuser which effectively extends the length of the underside of the car so that the travel distance is the same under as over and no lift effect. stop those frigging radiators from venting air down and under the car. all that kills lift.
next step is to add downforce - wings, front canards, vent the radiators upwards. vent the wheel wells (wheel wells accumulate air with a high pressure point at the top of the wheel just forward of the wheel centre line - vent the wheel wells up and you convert lift to downforce). beware downforce - the faster you go the better it works - chicken out and slow down and you lose downforce which means you lose traction ...
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