At high speed you can increase downforce and thus grip at the cost of increased drag. At lower speeds your limited to the tires which are effectively independent of the rest of the car. At high speeds wind resistance eventually dominates so:

Top speed ^3 * cross sectional area * drag coefficient = HP.

Which is why supercars are so low to the ground, having 1/2 the cross section effectively doubles your horsepower. Though that’s assuming the car can handle the forces involved.