Not trying to be a smart @ss-- but by the time you go through the process of shimming one of the springs, you may be close to the cost of replacing them. Last time I replaced rear coils it was about 85 bucks, I did the work myself. The only way I know of getting more out of a spring is a device that ya put in midpoint of the coils and it spreads to push the spring. Use to use these on front coils for sag. I don't know if they are made anymore and they did look like hell. Hope this helps.
No offense taken.... I understand your perspective, but I've had a lot of experience with aftermarket springs from earlier restorations. The problem is that they come in limited selection of rates (which affects ride) and a limited selection of heights (which affect elevation of the car). As a result, they are hit/miss.
My original springs are fine shape, provide original-quality ride, and (except for a very slight side to side difference)are very good on the height. Remember this is a 30K mile car that has never seen salt. It is, except for the passage of time, as close to a one-year old car, as you will find.
What I should clarify is how slight this height issue is. The amount of shimming I'm looking at is probably on the order of 1/8 to 1/4". The car has been this way for the eight years I've owned it, and may even have left the factory this way. It is very subtle and I've NEVER had anybody notice it or say anything.
So why not leave it alone? Well, I may do that. But it is just a bit of tinkering that is part of "dialing in" the car to my satisfaction. (Yeah, I know I'm nuts, but I didn't need this forum to tell me....my wife reminds me regularly!)
About the spreader devices you mentioned. My experience with those has been that they are more for preventing collapse of the spring when loaded, than raising the car when unloade.
As for cost/time.....If I used that logic on my car projects I would problably not initiate most of my projects. This is really about the quest for personal satisfaction, so if it takes all day to remove a spring and install a $1 spacer, I'm OK with that....if it provides the desired results.
I was hoping that there were some commercially available spacers (shims) for small height adjustment.