I just want to add my own comments to FreeFly's about the Koni FSD's. I've expressed my thoughts about them in other posts but not a sticky.
Basically, I agree with almost everything he said. I am also using the OEM springs, which I personally think are just about right for road use by enthusiasts who push the car hard. 1st track test will be in May, but I'm encouraged so far by what I'm feeling on the road.
The car is just so much more controllable and has far greater stability over bumpy back roads than it had before, by an order of magnitude. The crazy bounding up and down the stock dampers allowed is almost entirely defeated. There just is no comparison. I can safely drive substantially faster over the same section of familiar roads that sent the OEM setup into kill-me mode. On smooth sections, the car is very nicely balanced with some increased (but still smooth) compression damping that helps with pitch, roll, and brake dive.
Describing them precisely is a little bit difficult. The FSD valve is constantly adjusting the valving in response to the velocity of the damper shaft. It's a basically a trick new twist on a dual circuit damper. According to Koni, they need adequate suspension travel to work properly, which rules out most aftermarket springs. If you hit the bump stops they respond differently than a standard shock design - the FSD valve reads that as a high frequency bump in the road and relaxes the damping, which is exactly what you don't want. Fortunately, they almost never hit the bump stops unless you misread the road and hit a bump way too fast. Keep in mind that if you hit the same bump at the same speed, the OEM shocks would simply launch the car into the great unknown. So while it may seem like a weakness of the FSD design it really isn't (Lamborghini doesn't have a problem with it), you just have to drive the car properly. And even if it does happen, they recover very quickly, more so than the OEM dampers did, so the result is far less severe.
How do they affect the balance? Not very much, IMO. Balance, on a smooth road, is generally defined by the car's design, spring rates, and sway bar stiffness. Since I didn't change that, the balance is pretty much exactly the same - low speed understeer that goes away with speed, becoming more or less neutral at 50-60mph. And because they control body motions so much better, that balance is much easier to modulate. Result: more speed, more fun, more safety.
Ride comfort is in the eyes of the beholder. Many people mistake big bump compliance and absorption of high frequency road chatter as float. It isn't, in fact it's what a well designed suspension is supposed to do. You don't need to feel every invisible ripple in the road to drive the car very quickly. In fact, the suppleness of it's high frequency mode actually improves traction by allowing the tires to stay planted. Choppy roads that made the stock setup skitter around are a non-problem now, so you can maintain more speed and get better stability, especially under braking, if you hit some crumbled roadwork or choppy pavement.
Unlike FreeFly, I find them to be a bit harder over certain types of surfaces and smoother over others. Not by much either way, but the payoff is greater speed, stability, and control. Then again, it goes over urban speed humps and long neglected potholes much more comfortably now. So, overall, ride comfort is pretty much a wash compared to the OEM dampers. Less busy on smoother roads, but a little firmer over dips and bumps.
Ride quality however, is a different thing. To me it describes how well a suspension can deal with a wide variety of conditions without behaving badly or being too firm to be comfortable. This is where the FSD really shines. You get a big improvement in handling across the board with little to any comfort penalty. Mostly, the car just does everything better.
Some installation issues to watch for: 1) Koni puts way too much paint on the fronts, where they seat into the knuckles. Your life will be much easier if you simply grind it away with a wire drill brush and use anti-seize so they don't get stuck there. 2) The rear damper bushing/bracket is too narrow for the receiving bracket, using washers to fill the gap is recommended. You can deform the clamp enough by over torquing the bolt, but time will tell if that works in the long run (it's what I did). 3) The front end will be lifted a bit. That will settle down a little over time but my car sits slightly high in the front. I suspect that's good for handling though, and it certainly gives you more travel, which the FSD's like. In fact, since I carry so much weight around for my work, I'm looking for a way to mod the rear spring perches to compensate and make it level.
To summarize, IMHO this is an excellent change for those whose priority is real-world ride and handling. It is not a just a mod, it's a substantial engineering improvement. It definitely isn't the raciest setup, but they solve all the major problems with virtually no downside. More aggressive setups are likely to shave some tenths off a lap time, but this isn't a dedicated track car or autocrosser, it's just my road car, and the FSD's just made it better in almost every possible way.
BTW, to the guy who said it was developed on the Ring and needs no improvement, you're absolutely wrong. Mazdaspeed did the bulk of their testing right here in California, mostly at their own track, Laguna Seca. The stock dampers might do well there but in Real World, they just don't work all that well. All else being equal, I don't even need to guess that the FSD's are a big improvement over the stock dampers. Which, mysteriously, are absolute crap for ride and handling on actual roads and city streets.
Anyway, what I've been trying to say is that the Koni FSD's solve the main problems with the stock suspension, and will yield a huge improvement compared to the the driving dynamics compared to stock. IOW, you simply get a better version of the same basic car.
What you won't get is ultimate handling, because you've only changed the dampers. Better tires would help, but if you have the money and really want it to handle, regardless of the consequences, there are many options for that. I just know that the FSD's are very good ROAD shocks, and the increased performance they provide, without compromising ride comfort for the most part, is a huge improvement over the stock setup.
IOW, if you just want to fix your bouncy MS3 without punishing yourself on your commute, I think they're a very effective solution for the money, all things considered.