The first week I had the FSD’s they were used in conjunction with Nitto Motivo tires, including a trip from Franklin, TN to Blowing Rock, NC. I immediately noticed an improvement in ride quality in the areas I had hoped for. Impact of expansion joints, edges of bridges, speed bumps, etc. all improved markedly. The only significant improvements in handling I noticed was greater predictability and linearity on extended sweeping turns including off-ramps due I think to better roll control, and a slightly improved and more direct turn in and steering when making lane changes.
I needed to replace the Motivo tires which had worn prematurely on the inside edges due to an alignment issue. I replaced the Nitto tires with Michelin Pilot Sport AS3+. I really didn’t notice any notable differences in the two tires except that the handling just felt a bit more precise, and the ride comfort may be very slightly better, especially in differences in the coarseness of the pavement.
That same week we returned to New Orleans for a memorial service for my aunt. We had visited NOLA just a few weeks ago in August, and actually that trip was the reason I decided replace the struts. The pavement in much of the New Orleans area is patched, broken, or aged. We spent a full day in the Kenner and New Orleans area, being treated to a tour and history of the area by my cousin, a life-long native of the region. But by the end of the day the ride of the XC60 began to wear thin, and I actually felt sorry for the rear passengers. The return trip however was a different story. The FSD in combination with the Michelins mitigated or eliminated most all the harshness, especially noticeable in the rear suspension.
I-840 near Nashville between Franklin and Dickson is another area where marked improvement was noticeable. The pavement immediately on each side of many of the bridges in this stretch has settled into a dip. This has been a source of irritation to many, and in my opinion the State of Tennessee should be seeking remedy from the contractors for this stretch. Indeed, I once saw the contents in the back of a pickup truck launch from the bed of the truck up to the top or the side rails when driving across one of these bridges, and the rear wheels of the truck almost left the road. We had come to dread this stretch for this reason. However, with the FSD’s all but the worst of these spots were present, but well managed.
I now find the XC60 to be much more enjoyable. We love this vehicle, and have found only two things we didn’t like; the ride quality, and power rear hatch. (I much prefer the manual hatch on our 2004 XC90 with manual struts as the power hatch is glitchy, and sooooo slooooooow; a high tech and expensive solution in search of a problem, AFAIAC.) The installation of the Konis has eliminated one of these issues.
I find the Koni FSD ride/handling tradeoff to be optimal for my personal expectations for a sporty SUV at this point. Previously, the handling was great, but it came at the penalty of poor ride whenever the road surface was unusually rough, coarse, or broken. Installation of the Konis has improved the ride considerably. Normally increase in ride quality is at the tradeoff of handling, but these ride quality improvements not only retained the excellent handling but slightly improved the handling as well. Improvements in ride though, were much more pronounced than the more subtle improvements in handling. If you are seeking ultimate handling at the extremes, the FSD might not be for you and some of the Bilstein based options might be more to your liking. But if you are seeking excellent handling while retaining good ride quality, I have found the FSD to be a great solution.
I also think that they are equal or superior to either of the 4C suspensions I have owned, regardless of the 4C setting chosen. Some of the other members with FSDs on various other cars have talked about how their results varied depending on specific chassis and or springs. I can’t speak to how the FSD would perform with stiffer and/or shorter springs, as some others have done. But on this particular platform with the stock RD springs, I think Koni pretty well nailed it for the optimum ride/handling balance.
In summary, IMHO I think the Koni FSD are the shocks and struts the R-Design should have come with from the factory.