So the R6 shock worked fine (for me) with the 650lb spring fitted (12.5 stone rider).
Next up was the longer 2010-2011 BMW S1000RR shock.
I removed the R6 shock, took off the heavier spring, rearranged the spacers on the top clevis & transplanted the 650lb spring onto the S1000RR shock & finally fitted it. I had to the crank the preload right up to get it close to the right ball park sag for me: not sure why this is the case - different internal damping from the other shock, or is the length of the shock also a factor?
Anyway, after some initial testing, I decided to fit a heavier 700lb Faulkners spring to it instead (not enough room to use the TDM's spring). A 675lb spring would probably also be fine, but I couldn't find a cheapish one available when I went looking.
This worked well for me. Of the two shocks tested so far, I actually prefer the BMW one although difficult to articulate why - somehow the rebound 'feels' slightly better.
Pics to follow.