My own experience with FSDs is 100000kms over seven years.
I know no-one across any of the A2 forums that currently recommend FSDs over Monroes, Sachs, B4s or B6 / B8.
Koni's Quality control was awful. The consistency between batches is crap (feel free to read up on that one across the forums). The failure rate of FSDs in real life is higher than any other damper I've heard of, mainly because I've heard of several damper failures here and several replacements over in central Europe. I know of no other damper where this is the case. That aside, the consensus is that FSDs *will* raise the ride height of the car by around 15-20mm, that they work best with a little pre-compression, and that they're not OK with serious lowering. You are limiting the freedom of the damper to move at the top end, limiting the suspension travel itself because of the physical damper size. This is why B8s should be used >20mm lowering - physical size. So by pushing the piston in permanently, you're limiting the ability of the damper to work correctly and effectively.
Regarding the "not made for lowering"; Once upon a time, the statement "FSDs are for max 35mm lowering" was on the Koni site. I think I can dig it out of the German A2 forum, because I remember checking it against the 30mm the Eibachs were supposed to give and it was supposed to be OK. The quote from Schnelletrecker from the dawn of time (2007) is here:
http://www.a2oc.net/forum/showthread.php?4807-In-search-of-comfort-Koni-FSD/page2 . Post 53 from that thread has more information and links, including this one:
http://www.a2oc.net/forum/showthrea...mpers-coilovers-and-airride&p=45498#post45498 which is rather good. That thread is also worth reading.
There is no longer a lifetime warranty on FSDs, and anyway, you'll have to have them professionally installed if you have a warranty claim, and even if you do, you'll need to send them back to Koni in Holland for them to check over.
Pictures of mine will be online soon. Rust on the outside of the cylinder at the rear, flaking paint on both lower mounts at the rear. Flaky spring mounts at the front on both sides; another 12 months and they would be at failure point.
I cannot recommend FSDs and I would not recommend combining them with anything other than stock (or similar) springs if you want to take advantage of their functionality.
- Bret