Consider this then. In order to lower the car, the spring's length has to be shorter. That means the weight of the car will compress the shocks until it meets the top of the spring and the resistance holds up the entire car. Shocks were not made to be partically compressed 24/7 more than it was origionally meant to. This would also be what leads to faster wear on the shock itself.
Going back to the aftermarket spring, they're shorter in overall length so the car can sit lower. This is the obvious. They say it's made to work with the stock shocks, okay, but think about it. It doesn't make sense. If it's made with the same rate as the stock shocks, then you would bottom them out (one reason why cutting stock springs are bad) on major turns, braking, or on acceleration. The only way to compensate for that is to make the spring stiffer, and thus avoid bottoming out the shocks. But if the spring is stiffer, it no longer works with the stock shock.
Maybe there's some spring manufacturing secret that I'm not aware of and defies the laws of physics... I guess I did get a C in physics for engineering majors
I've been reading some stuff and I'm quite impressed how Whiteline is not hiding the facts about lowering springs. Take a look at this link, it's also full of great information.
http://www.whiteline.com.au/default....faqsprings.htm