Just read this old thread with interest. I fitted an RTC steering damper to my 2005 110 SW about a year ago.
lt's been all good for me. The Defender is easier to manoeuvre at low speeds, while at higher speeds it seems no different.
When taking a corner on the road the steering has a better self centering action, making it easier to drive.
The spring has never moved although the tube has a little surface rust after 12,000 miles (now sprayed with grease and has got no worse)