You also need to adjust your transmission if you need top speed. lets say you have a gear ratio that in theory would get the car to reach 330 km/h, the 330 km/h will not be reached because you have other facts like friction from tires, downforce etc that make the car only reach let say 315 km/h.. but if you took away all downforce you might reach 325, the tire friction can not be taken away im afraid :-)
Maximum downforce is a great tool to optimize time round a track. However, the track layout desides if you are going to hava alot or none downforce. As monoha says: High speed circuits=low downforce, technical courses=high downforce
tuning a car is complicated and requires some practice. You'll figure it out soon if you dont get it now. Just try 2 races. One with max downforce and one with no downforca around daytona oval and see for your self. The car can not reach the rev-limiter if this test should shine some light for you.
Sorry for my bad english, hope you guys understand my point