Boiling it down into simple terms, there's no point in turning an engine past its appetite for air. More power will not result because the engine is on the downhill side of the power curve. However, if you install a head that flows more air, then you have to turn the engine at higher rpm. However, if the valve train and bottom end can't handle the higher rpm . . . broken parts in the
oil pan . . . then there's no gain. Better yet, keep the revs at a safe level, but have a valve/port that flows better at lower lifts, then you'll have improved torque and horesepower at a tolerable rpm.
About increasing all out vehicle speed, be aware that aerodynamic drag increases as the square of speed. Regardless of power, a car such as the Cobra that has the aerodynamics of a brick, is not a high-speed car. That was the reason Shelby developed the Daytona Coupe. Make that sucker slick and it will achieve a much higher speed with the same power. The GT40 is another excellent example of this. One thing we know from running at Bonneville is it's better to work on aero along with building more power.
On the other hand, power and the light weight of the Cobra is what kicks you in the rear at lower speeds, or less that 100 mph. That's the delight in driving a Cobra. So go for power, but at a safe rpm.
