if you look at saloon car racers from the seventies (batmobile bmws, zakspeed fords etc) when they experimented with these things, or the pikes peak racers, you will see that they have huge appendages (ooh er missus) on to generate meaningful downforce. that tells me that the basic shape of a saloon car is not suited to generating downforce.

as for your first question, the idea behind downforce is to effectively add more weight to the tyres to make them grip better, without adding mass that makes braking and acceleration worse (ignoring the fact that to generate downforce will probably increase the drag so your high speed acceleration will suffer).