I’ve had it on for about 3k miles and, I don’t measure the stance regularly, but I have not had any problems with it at all, here are some recent pics, this is probably after about 2800 miles..
I have the pro-kit on my 07 and it works great. The only sagging issue I have heard of with Eibach is the sportline kit for the 87-93 fox bodies which I have them on my 89. When looking at the car it looks to be setting lower in the rear but when measured it is actually dead even with the front.