Of course, Per is right on with his assessment of the 1212. The current production model is really more of a full 12-weight. It is a tad on the heavy side for 12-weight heads, although I do find I can get it going pretty good but it does require a bit of oomph.
I have a second prototype with a more radical taper and a lighter tip which throws short 12-weight heads very nicely. It was my initial understanding that the final production model would be more like this one. Unfortunately after several fly shows where the A prototype was shown around, a good number of orders were placed for the rod. A decision was made that the rod couldn't logically be changed due to the existing orders and the excellent reception of the first rod at the shows. Of course, at the shows, the rod was being demonstrated with a much heavier sinking head and a thin running line, so everyone was getting giddy casting 150+ feet.
I have recently heard that the folks at, ( Insert name of big western US fly line manufacturer where British champion spey caster works,) also much preferred the 1212-B model, so maybe with a bit of conspiracy the production model might be changed.
If that happens, guys like my friend Per will come out of the woodwork saying, ' You screwed the rod up, now it won't handle the heavy lines the way it used to!'
For now, I would say that the DH1212-3 can be used with either 12 or 13-weight heads. The 12's require more casting effort, although you can work them confortably at 100'. I know some guys who are overhead casting sinking heads as heavy as 650 grains on them but those feel too heavy on it to me.
I also started using some shorter light line rods last year, the T & T 1208 and the Redington RedFly 12'6" which is an older Loop design ostensibly rated for 7/8 line. I find that both of those rods will work various shooting heads of 9, 10, or even 11-weight and they are a lot of fun in protected estuaries when the wind isn't blowing too hard and the flies aren't too big/heavy.
I think Juro has a lot of experience with the Sage 12'6" so maybe he will comment on that for you. In fact his Sage was the very first two-handed rod I ever cast one warm December day a few years ago on Race Point. One cast with it and I was hooked on two-handers. Thanks Juro!