OK, the thing to understand here is what shock and springs do for your suspension and handling. Shocks/struts are on your car to control the motion of your springs after traveling over bumps/dips. Springs are there to maintain the tire's contact with the road when encountering bumps and dips in the road. The also help the car resist body roll in turns. Ever watch the movie Uncle Buck? There is a car without shock absorbers, just springs. The thing just never quits rocking and bouncing around. Shocks basically have rebound(downward movement) and bump(upward movement) that is controlled hydraulically, with the use of oil, or pneumatically which are your air/gas shocks.
I assume you are looking for the best setup for high performance driving and not necessarily the most comfortable ride.
On springs you want them as stiff as you can without the car bouncing off of bumps. If they are too "soft" the coils will slam into each other causing too much body roll, slow steering response and, when they fully comprress, the front will start to push or you will lose control. If the springs are too hard the tire will not be able to comply with the bumps and dips in the road and will the car will bounce off of the road causing a loss in traction.
For shocks you want the shock to be stiff enough to keep the oscillation of the spring(bouncing up and down) to just one cycle. If the shock is too "soft" the car will bounce around and be move a lot during braking, accelrating and cornering. This also messes up weight transfer and causes you to lose traction. If the bump is too "hard" it will keep the spring from doing it's job of absorbing the bumps and if rebound is too stiff the tire will not fill a dip in the road. Too stiff of a shock and the car will be very hard to control over bumpy surfaces. So it would be a good thing to have adjustable shocks where you could adjust both rebound and bump(Koni makes good ones).
Your GC coilovers are also designed to provide you with the ability to change the static weight distribution of the car. You are able to put more weight on a wheel by lowering the adjuster for the spring on that side or by raising it on the opposing side(diagonally). You use them to not only lower the center of gravity of your car but then you can manipulate it to give you a very balanced response, as far as how weight transfers, in your car.
Other things you can do to tune a suspension would be the addition or subtraction of anti-roll bars(minimize body roll, keep tires on the ground during cornering), changing tire pressures(keeping the most contact patch on the ground), adjusting camber(maximizing grip during cornering by making a flat contact patch), and adjusting caster or toe(helps to increase steering response or stabilize the car in a straight line).
Basically, it takes a lot of playing around to get it just right and it does depend on how you drive and where you drive.
Also, a little caution here, adjusting these things I mentioned above beyond what is stock can do things to make a car more unstable in certain situations(such as increasing toe out makes the car more unstable in a straight line) and can lead to increased or uneven wear in you tires. If you're not sure, what will happpen, read some more about it or talk to a professional.
