the Bilstein Sports are just built for the shorter travel of a lowering spring while the HD aren't. So like they said above it's the same internal valving but made optimally for a shoter spring. Other thing is that Bilsteins have internal bumpstops incorporated while other companies have external bumpstops that one has to replace or cut if going lower.
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFOLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA SPORT TUNE - AA PULLEYS - BILSTEIN PSS9s - AS 40% SSK
CF CSL RACE LIP - NAVI - BMW PERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC SS BRKLINES, TRANS MNTS, RTABs - 4.10 M-SPORT DIFF