Well let's see...

Ride hide adjustment
No bending of coil spring so more accurate spring loading
available in lots of different valving setups
some are adjustable, really nice, especially for someone who works on the suspension like you
Availability of progressive rate springs
Easy to swap springs
Rebuildable units last a lifetime
Available with rod eyes, n deflecitn bushings, the stock top mounting on the front is a joke
the cool factor

 

Ride height , sure, found how nice it was on the back this week to just take the special wrench and raise or lower the car.
What do you mean no bending of the coil spring so more accurate spring loading? That little coil over spring is stessed alot more then the big spring.
Choosing a valving is real difficult and once choosen you are stuck with it. I spent $730 on a set of Pro none adjustable for the rear. I have one chance to choose front rates or spend about $1100 for adjustable ones. Not cheap. I can buy normal shocks alot cheaper.
It is easy to swap springs but at $140 a pair not cheap so not alot of testing but it is easy .
Sure they are rebuildable but once agian I could buy alot of normal shocks for the price of 2 good coil overs.
The ends ,I could make anything fit if I desided to go that route.
Twin turbo I have a good rate of front spring #280 and good shocks that I like, the ride quality is good and the car doesn't roll over too much on cornering. While the idea of coil overs appeals to me the cost of just saying I have coil overs doesn't . I would spend in the $1100 range for the front. Even in Canadian dollares that is alot.
I am also thinking about 20 inch wheels because I found the rims I like and they only come in 20 inch sizes. I am concerned about the tires available.

 

The other benefit that has gone unmentioned is the ability to adjust the load on each corner of the car. Norval, talk to your circle track buddies about cross weight, fore/aft and left/right weight percentages. It plays a significant roll in cornering performance and balance. With coilovers and semi-coilovers you can make changes easily to find the best combination.

 

On the rear I can see cross weight for increased traction but for a car on the street going around corners left and right how would I cross weight. I would assume I would want the same on either tire.
Speaking of weight on either tire I could put a set of scale under each front tire and adjust them to equal each other. I do know that my motor is up to 2 inches shifted to the passengers side to offset my weight but are they equal?
Never thought of that. Thanks

 

You are correct, for a street car you want it basically equal left to right. Fore and aft are a different story. It would depend on the overall setup of the car including weight distribution. Yes you could add coil spring adjusters to the stock front end and accomplish the same thing. Rather than moving the engine I would think you could shift the battery, alternator and maybe a few other things over to the pass. side to help equalize things weight wise.