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 AUTOFLD MIRRORS - CSL RACK - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAV- BMWP CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC S/S LINES, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF