To be honest, Eibach tries out a ton of spring rates and tests them fully.
BG does one to 3 sets usually and sees which goes best with the car, most of the time they get it spot on the first try but when it came to the cruze, it took months and multiple fittings.
We like both companies very much, we had one of the first Eibach production test sets on our Cruze, we really liked them a lot. We also have a set of BG springs on a customer car as of last week, we are waiting to hear back from them.
We are keeping a close eye on what is coming out for the Sonic, we have also contacted many companies trying to get new parts as fast as possible.
Thanks
Steve