Yes, a truely useful spoiler creates downforce at the expense of drag, witness NASCAR spoilers, when adjusted for downforce, they decrease top speed. However, if you have an inefficient aerodynamic design to start with, you can increase downforce and decrease drag at the same time. Using NASCAR as an example again, when the cars qualify for a race, the radiator inlets are taped over, which decreases drag and increases front end downforce. Of course, you can't tape over the inlets except for very short (probably under 10 laps) runs because the engine will overheat, which explains why those holes are there to begin with.
Zenmervolt