Unless you have access to a wind tunnel or some serious track time and datalogging equipment, you'll never really know exactly what wing size you need for high-speed. Also worth bearing in mind that they're needed to generate downforce through a high-speed corner, not just a straight line: Look at the Veyron when it goes into it's max speed mode, all spoilers stay firmly down and it drops itself to the floor. That's what you're after for high speed stability in a straight line.