Aerodynamic Drag
Engine power is not only used to overcome tire rolling resistance and drive train losses, but also aerodynamic drag. The HP needed to overcome aerodynamic drag is related to the cube of the velocity. So the power consumed at 200 mph will be eight times more than that at 100mph, and at 300 mph, this increases to twenty seven times. In other words, if it takes 80 whp to drive a car at 100 mph, this increases to 2,160 whp at 300 mph. If you add aerodynamic loads (drag) to that same car, it will require more than 80HP to maintain 100 mph. One needs to be mindful of this. If you install huge aero devices on a car that does not have the power to support them, it will not be able to acheive the same top speed as it did without them.
Front-to-Rear Downforce Balance
The aero downforce created by the wing at the rear of the car must be balanced by downforce at the front of the car. Excessive rear downforce will cause high-speed understeer, so you can only use as much wing pitch as your front downforce will permit. A very effective front aero package can't be used with a so-so rear wing, as the wing won't create enough downforce to balance the racecar. The balancing needs to be done for the highest-speed corner on each track you race at. So adjustability of the whole thing is essential.
Fully Developed Aero Package
It is best to think of this as a process than a single goal. At first, you will be looking for improvement in lap times in the range of seconds. This will be reduced to tenths of a second as the aero development progresses. Once the optimal downforce level is reached for a particular track, lap times will not improve or may even increase as more downforce is added. For the club racer with a limited budget and running in a class where horsepower is limited, the process will be very different from a team competing in top-level open wheel racing. The universal goal of lower lap times continues to be met at all levels of motorsport, through a combination of hard work, ingenuity, improved engineering tools, and advances in materials and processes. This is what race car development is all about. If you truly want to be competitive, you'll need to step up.