The sag will change the geometry in a different way than the ride height. I don't think you have to consider it though. Just set the sag to 30MM and play with the ride height (from the stock setting) just if you want to change the bikes handling. IMO.
Actually my ST3 has no adjuster. I'm trying to prioritize, in the middle of winter, the few modicfication dollars I have, and a ride height adjusting linkage arm's on the list, but, I'm not sure how it might affect the shock's ability to compress/rebound once extended to the recommended 1-2cms gap between the rear tire and the ground. IIRC it doesn't affect pre-load, but I though travel may be a different issue considering the progressive rate linkage, and I wouldn't want to reduce the shock's ability to travel fully within its design paramaters in exchange for quickened steering. My concern is with rear tire traction, so I guess my questions are, "Will rear tire traction be compromised as a result of reduced shock travel when the rear end is raised using the adjusting arm?" and, "Would I have to rework both preload and damping everytime I raise/lower the ride height?"
(but then again, as Liquid points out, I use a "laidened" sag figure, not a "racing" sag figure so maybe it wouldn't be an issue for me. LOL!!)
Frank, I highly recommend that you consider adding the ride height adjustment to your ST3. For the money, it is a very big handling improvement. I bet you can build one yourself or have a friend with some metal working machines work on it with you.
I think you are being very astute in being concerned with the geometry change vs. linkage position. As it is some type of (razing?) rate linkage your going to start in a different part of the range. What I think you will find is that the effect is rather minimal in the overall scope and won't likely be a practical issue. Isn't the ST3 sort of patterned on the 851 and even Monster type geometry? I personally would not want to make big changes to the trail by razing the rear by huge amounts, it is an ST rig and a little fine tuning won't hurt. The 851 guys often raise their bikes up a lot.
Not having a lot of experience with exploring the relationship between ride height and progressive rates in a system where the ride height adjuster is part of the progressive rate linkage, my concern is with traction. I can't help but think that "something has to give somewhere" in terms of the shock's compression/rebound travel in a system which AFAIK, is designed to compress and rebound from top and bottom at the pivot points, as opposed to bottom only like conventional set-ups with one point fixed.
AFAIK, the ST3 frame is the same as or descended from the 851, and has fairly steep geometry for a ST, 24 degrees, same as the ST4s, so, there doesn't seem to be to many complaints about rear slides, *or* (amazingly) tankslappers from that crowd, who mostly rave about the "balanced handling" once the rear's jacked-up a bit, and I guess one should trust the factory re possible safety issues, ie they wouldn't have one if it posed too much risk, right?.
Now, another issue: It's money spent, but no bling factor. But not too much from what I understand, so, I may add it come spring if I can find a good alternative to Ducati's higher priced version.