For those of you who have installed these springs, can you comment on the advantages and drawbacks as compared to stock? I have a base C2 and I am considering installing these to add some stability at higher speeds. Did you notice any improvement in high speed stability? My car does not inspire confidence at speeds over 80 MPH. It feels too busy and twitchy. Did the springs help this?

 

I am curious as well.

 

I got full coilovers upgrade on my 991s, I lowered my car around same range as lowering springs.

I can comment on Pro and Cons:

PRO:

Lower center of gravity
Better handling
Better steering feedback (with Coilovers in my case)
Looks way better
More stable at high speed, lower COG

CON:

Ride is stiffer (If you have PASM is like SPORT Mode on all the time)
Long road trip more bumpy and more tiring
You have to go into steep driveway or speed bump at angle to avoid scratching the lip
You have to go super slow over speed bumps
You might scratch/damage your under panels if you go over speed bumps too fast.

If your car don't have PASM then your stock ride height are pretty high, lowering the car will help quite a bit in stability and cornering.