I agree with EB spares on this. If you think about how the FSD works, i.e. it is basically a shock that has 2 stages, a certain amount of travel within stage 1 at a reasonably soft level, to absorb a large amount of bump/rut/pothole, and then a second level in which the travel is stiffer. By lowering the car via springs you are using up a significant degree of the overall travel of the strut, i.e. you have compressed it, and as most of that travel you have removed is part of stage one, thats where the improved ride quality is. So, and I wish I could find the words to explain this better, basically, its obvious spring choice, and therefore ride height, is crucial when fitting FSD's. Too low is bad, very bad. Might look cool, but ride/handling has always been a compromise, and now people are adding the looks of the car into the equation. Something has to give, and its usually ride quality. I personally think the FSD's with Alfa sportpack springs are the best compromise, but thats just me. Alfadoit, maybe you should fit your original factory springs, if you still have them, I know it'll cost you bucks, but it will save you from buying new springs and not knowing how they will effect the situation. And stop taking your frustration out on Pud, he's only trying to help, same as everyone here. Pud, what Eibach springs have you got fitted, I believe there are 2 types, maybe this is where the differences/confusion lies.
I used Eibach Pro springs, on a 156 they are only 10mm lower (-30mm) than the optional Sportpack springs (-20mm). Too low is bad on any shock thats not designed for it, but that is the good thing about the Eibachs - they don't compress much once you get past the inital progressive part of the spring. You'd probably be more likely to bottom out the shock using the slightly longer but softer Sportpack spring as it is less resistant to compression across most of its range.
We fitted a few GTAs on standard springs with FSDs when they first came out and driving back to back with ones with FSDs & Eibachs, the Eibachs were the better of the two.
I wouldn't want to fit Sportline springs (-50mm) with FSD shocks, Bilstein B8s would be a better option as they have a shorter rod with much shorter springs in mind.