Here are the pros and cons of the GC system as opposed to lowering springs as I see it.

Pros:
-Can adjust height to exactly where you want it
-Height adjustment is good for ground clearance issues with future headers
-Never have to get the springs compressed to change shocks again
-If the springs sag more in the future, you can just adjust it back out
-Installation is very straight forward

Cons:
-Have to keep adjusting it to get the perfect balance, but I got mine within 1/16"
-Springs in the RKsport kit are about 30% stiffer(400/180) than stock. It's slightly harsh but tolerable for daily driving. Non-stock shocks would probably eleviate that problem.
-Springs are linear instead of progressive rate.
-Weld in rear spring collars

If you decide on the GC system, make sure you purchase an adjustable panhard bar when you install the springs so that your rearend can be readjusted to the center.

I would suggest grinding a small blend radius into the lower cutoff of the spring that contacts the adjustment collar. It has a tendancy to bind at the notches in the collar. I would also glaze the area between the collar and spring with some grease or antiseeze to help tighter adjustments.

Do the fronts first, run the collars to the same number of threads from the bottom. You'll need to put the tires back on, lower the car and drive it back a little and then forward; otherwise you get a jacking effect from lowering the car one side at a time. Take measurements, then adjust the collars. Each revolution of the collars is worth about an 1/8" worth of height travel. I would suggest adding an extra 1/4" height for the springs to settle into.

When doing the rears, place the lower cutoffs of the springs facing to the left, so your adjustments can be consistent. Look at the lower portion of the adjustment collar to see where the thread starts and mark that position on both collars. Then if you reference both those marks to the back of the car and have both cutoffs facing left, your adjustments should be very close to each other. I would start off with the same number of revolutions, drive the car over some speed bumps to get the springs to seat,
take measurements, and then make your final adjustments. I would suggest adding an extra 1/4" height for the springs to settle into. You'll need to drive it over bumps again to get a reseating.

Once you have all your heights adjusted, get a string and plum bob, hold the string against the outer fender well, center the lower portion of the string to the center of the wheel hub, and take a measurment from the string to the center cap. Once you have some numbers, adjust the panhard until you have the car centered fenders to wheels.

When you are all finished, give the springs about a week to settle and then take the car for an alignment.

Side note: if you decide to use lower control arm relocation brackets such as my BMRs, don't use the lowest adjustment location. That gave me really messed up rear adjustments and will drive one bonkers.

Good luck

P.S. Eventually I will do a write up with pics for this and submit it to www.projecttransam.com.