Standard rear shock has performed well for me, most of the time. Many interstate trips two-up, and towing a trailer, so often a fair load on the rear end.
I have had a problem with rear tyre wear, pretty severe scalloping. Another ST1300 owner, Trav (on the Facebook site) had a similar problem. There was some discussion elsewhere (dunno where it started) about the pre-load adjustment mechanism needing a service - if not, it caused the scalloping. There is some discussion on this link -
https://www.facebook.com/groups/ozstocgroup/permalink/655499245050874/?comment_id=710158279584970&reply_comment_id=710160289584769 - hope it works for you.
Anyway it seem to have solved Trav's problem. I have just serviced mine, but have not had a chance to try it out as yet. Before my servicing, the pre-load had no "feel", post service it has a good "feel" - by that I mean, as I wind it "in", I now feel some restistance. Anyway, fingers crossed.
There's also a Youtube video, or three, on how to do it - here's an example:
https://www.youtube.com/watch?v=IgOWLfPiVQQ. If I can do it, almost anyone can.