You would like to have your shock operating in the middle of it's travel such that you don't bottom in compression. It is not uncommon for the spring to be loose at full droop. Very lite helper springs can be added to keep the springs from unseating. Limit straps are another system used and I use it on my Wrangler, but it would not be my choice on a Sports Car.
If the spring is too long, you end up with a short stroked shock in bump meaning you limit the shock before the suspension hits its limit.
If they are too short, they will unseat early in droop. What you would like to have, is at full droop there would still be some spring force acting on the perch. That is not always possible because of packaging. Nor does it usually matter because we don't run to full droop in a street car. That said, if they are unseating during normal driving they are way too short.
Eibach (as well as many others) make them. Summit will sell you some. Summit.
http://www.summitracing.com/parts/ei...r250/overview/
I went thru this on my FIA and was able to find suitable lengths that did not require helpers while optimizing the shock travel. IIRC 2 of them are not fully seated at full droop, but there is not enough room for a helper.
chr