I refuse to get drawn into heavy discussion about the springs but I will state my personal preference. Since it is clear dyno proof is not needed here, this is what I know from my personal experience...take it or leave it.
I installed the first set of springs in my Spider as one of the prototypes. I noticed a gain in the mid-upper range powerband. At first, I figured butt dyno maybe, who knows but it did pull hard. Several thousand trouble free miles later, I did one run on a 1/4 mile strip and laid down a 14.5 second time with a 98 mph trap speed. This is on a fairly mildly modified car, no meth, no slicks and no extreme prep. I drove 1.5 hours to the track, did a run and drove home.
Now, either I am an incredible driver (not) or the springs made a difference. Nobody else has gotten a 98 mph trap (that I know of) on a stock turbo. The big turbos are only doing 99 mph but I am sure that will change soon if it hasn't already.
I do not sell the springs nor to I endorse them, this is my personal experience. With what happened, it is only natural to be worried about running the new springs, I get it. I have not installed my new springs yet but I will as soon as I have time. When I pulled the springs out, I pulled some of the mid and upper range gains that I had gotten used to. Believe me or not, I understand and I understand the skepticism.
Each of you must do what they are comfortable with and I 100% respect that.
Stage 1+ tune with basic mods and springs