remember that with springs, it is important to have a matched set of dampers. The nice thing about coil-overs is that the spring and damper are matched from the manufacturer. You can do the same thing with replacement springs, but it isn't easy to find the right match. If you notice the Mountune springs make a note to mention they work with the factory dampers. This is a very important fact that is easily overlooked when people are going for the "moar low" look. You can increase spring rate, but need a damper to handle the increase rate. My Mustang spring rate is roughly 4x the stock spring rate, but with the right struts, it is not harsh at all. It sure is stiff, actually stiff enough that when I rock the car, only the sidewall moves. When lowering a car using springs, it is equally important to make sure the dampers have enough travel. Typically, lowering springs are too low and the damper crashes into the bump stop causing the shock or strut to fail. You need the right travel and dampening rates to work with different springs. An adjustable coil-over has taken those factors into consideration. When you mix and match springs and dampers, you rarely can get all the data needed to get the combo correct. If you buy a "package" that includes match dampers and springs, but aren't adjustable, you are getting a well matched system at a lower cost than coil-overs. The term coil-overs is kind of used incorrectly as it has been accepted that "coil-overs" indicate adjustable spring perches mounted on a threaded strut or shock tube. In fact, any MacPherson strut with a spring mounted on the strut is a coil-over regardless of adjustment since the spring is over the strut.
As for the best suspension setup? There isn't one. Suspensions are a compromise. What is best for an autocross course isn't the best for a race track and are not the best for the street and not the best for rally. The focus needs to be on what you want the suspension to do and pick parts accordingly. For me, my ST will be 99% daily driven so I want something that isn't harsh, too low, too stiff (gets slippery in the winter), or too compliant. That leaves stock or the Mountune springs as my best bet. For the "moar low" crowd, my choices would never be accepted. For the time attack crowd, my choice would be too compliant and too tall yet too low for a rallycross or rally car. It is all compromise based on what you want the car to be.