I installed FSDs on my 335i with sport suspension about a year ago. The improvement in handling on smooth surfaces is subtle but they really come into their own on poor road surfaces. They make a huge difference in the way the car handles pot holes, construction plates, frost heaves, etc.
I had no issue with the way the stock dampers, or the RFTs for that matter, handled the smooth roads of South Florida. When I had the car shipped to NYC the combination of the stock sport suspension dampers and the RFTs simply did not work on the war ravaged roads of NYC and the surrounding area.
I would highly recommend the FSTs to anyone who has an issue with the ride quality of the rather poorly sorted out stock sport suspension. Other than the cost there seems to be no downside with going with the FSDs.
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