Here's Bilstein's email response to my questions
 

This response is fairly similar to the advice I was given over the phone which I posted earlier, except this time they would not commit to a quantifiable difference between Sports and HD shocks. The questions I asked are below; Bilstein's email response follows.

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I would like to know:

1) Is the purpose of the gas fill only to prevent foaming of the hydraulic
fluid or does it actively contribute to the damping; if so in both directions
of travel?
2) How does Bilstein compensate for changes in shock temperature (either from
ambient conditions or racing) such that the performance is consistent? I
would think the gas pressure would change with temperature.
3) Are Bilsteins more stiff than say Koni's or Boge's because of the gas
charge or because of some other internal design.
4) Is there a quantified percent difference in stiffness between the Bilstein
HDs and the Sport Shocks.
5) Lastly, is there a Bilstein recommended setup for Porsche 911's (eg. HD
front, Sport Rear).

Your answers will be greatly appreciated.
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We were forwarded your email from our German parent company.
One thing we might point out is that Bilstein was available as original
equipment for Porsche 911, so the "standard" shocks may already be Bilstein

1) The gas fill serves to prevent foaming, and the (internally) separate gas
chamber serves as a compensation for the piston rod volume as the shock
travels through its stroke, as well as for changes in hydraulic fluid volume
due to temperature. The gas pressure does not actively contribute to the
damping.

2) The effects of changes in shock temperature on damper performance are
minimized by use of a hydraulic fluid with relatively low rate of viscosity
change over the temperature range. By their design, Bilstein monotube struts
and shocks provide excellent dissipation of the heat generated from damping
forces, since the working cylinder is directly exposed to the ambient. So
heat buildup is less than with conventional twin-tube dampers.

3) Both Koni and Boge have gas charged dampers. Differences in stiffness
result from the internal valving of the dampers.

4) For Porsche 911 the HD and Sport shocks are valved quite differently,
though there is no across-the-board percentage difference.

5) Our recommendations are in our application guide. Many tuners will
combine different settings front and rear, in conjunction with other
suspension modifications; however we do not make specific recommendations in
that regard.

You can contact Krupp Bilstein of America at 858-386-5900 or fax 858-386-5901.