Hi Jason,

Please remember, that such a system is not "active", it doesn't supply energy to the suspension.
It's a form of an "adaptive" damper control, which can vary the energy disipation in the suspension, according to some parameters.

AFAIK the VW system comes from Monroe [Tenneco], produced in Spain and uses Öhlins (CES) valve technology.

If you are interested in some of the in and outs, you may want to have a look at this thesis paper.
in more direct response to your question, step response time for the valve is between 5-25 ms [40 hz], but at higher frequencies, other factors, such as damper (body) stiffness and oil characteristics feature quite strongly as well. But up to 20Hz shouldn't be "a problem".

http://www.diva-portal.se/smash/get/diva2:539767/F...

to see the difference between a "fully active" and a "adaptive" (sometimes called semi-active) system, you may want to take a look at the following presentation, which compares Tenneco's "active" (ACOCAR) and their "adaptive" (DDC, CES etc.) systems, under different conditions.

http://www.ac-kolloquium.rwth-aachen.de/pdf/Vortr_...