It's more complicated than you think. Technically lower downforce means greater straight line speed. But it really depends on how much aerodynamic parts you have, and I'm not talking about F1, F1 cars these days have a lot of aerodynamic parts which makes them produce a lot of drag. Increased speed therefore is the best compromise in aerodynamics and power balance. But without downforce the car will become unstable at top speeds, due to lack of downforce. Although increasing the rear toe angle can help and it may be good if you decrease the ride hight and the spring rates.
This is true in real life, but in this game, if you are running like 200 mph and dont make sudden movements, you wont have any problems. Sure your car will be a litte bit more stable, but the person with higher top speed will beat you then. I have raced manny cars with spoiler etc, and the increasing drag makes the loose.
To make this clear, you DONT want to have any downforce while drag racing in this game, thats it. You also want toe and camber at 0.0 for maximum grip. I have drag cars that are really unstable at high speeds, but you need to drive calm and clean. The positive part is that they have good acceleratioin and grip, which is crucial in drag racing i would say. Then if you crash your car in game, nothing happens, you wont die, as in real life.
Some people also say that downforce increase the grip by pushing the rear down, well that is true, but in reality, those effects only work well after a certain speed, which is usually pretty high compared to the speed your cars is traveling when the tires spinn.
And, then if you race around the ssr7 then its circuit racing, in drag racing its only important to optomize your car for straight line.
Im not a expert, but as far as I know, and what i have seen, minimun downforce = higher top speed and acceleration, and that is important in drag racing.