Thanks for posting this, it makes a lot more sense than what I've divined from reading threads.
Can I use JDM springs on USDM stock dampeners?
...however I do not believe USDM springs can handle stiffer lowering springs.
Minor typo, should be dampeners
What about aftermarket Bilstein HDs?
The bigger mystery is the difference in price ... My guess is the proprietary dampening and stricter quality control.
: Price difference based on fewer middlemen / lower markups. In my opinion Subaru has very high part markups. For example, Brembo OEM front calipers through Subie are >$1000/pr, while similar OEM Brembos through Volvo are <$600/pr and a full Brembo front brake system (pads, calipers, rotors, etc.) for Mustangs can be purchased new for <$900.
The aftermarket Bilsteins likely have different damping rates and spec ranges than factory, but same quality control (different quality control systems would be unusual in manufacturing).
I wonder if they fit the USDM top hats?
just my $.02,