I found your thread during a search as I'm considering doing almost exactly the same thing, for my RX-8.
Thanks for spelling out some of your findings. How're things progressing?
Also, do you have a reference for these dampers being spec'd at 20in/sec? I see the SL series are spec'd at 10in/sec, but I don't see a decoder for other series. Either way, it's funny they spec them at this velocity, since most shock dynos for commercially available coil over packages don't go to 10in/sec, and none I have found go to 20in/sec. It's likely, this is the range where the differences in ride quality and road-holding come into play. My limited research indicates "coil over" manufacturers don't like to release the magic numbers because it's either their secret sauce, or the skeleton in the closet, depending on the make...
Also, have you seen Bilstein offers an adjustable shaft for their 46mm dampers? It can be single or double (simultaneous) adjustable, or independently double adjustable if you go with an external reservoir with compression adjustment. No clue on price...probably expensive.
Hope it's working out!