Sorry to hear you are having the same problem E91 M Sport. My noise has increased again since it has been raining and in the 60s all week. It's really annoying to hear this clunking while driving around.
Your issue may be different, but in my case I dont believe it's the spring. I think it is the valving inside the shock. I've tried use thick rubber tape like you linked to and it did not help. Let me know what you find out!
"Groaning" seems to be the right term and there seem to be lots of other cases of it with varying levels of support from KW. I've had my rebuilt and tested twice by KW without any success. They said there is no local support in the NC area so I'm out of luck.
http://www.golfmk6.com/forums/showthread.php?t=23060