Paul K, what you describe, it's all been done.
I have a 9 incher in my workshop. With 4 tons of mass.
http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double
Not saying that this is a perfect setup. But it's not too bad.
The only point i really don't like, it's the two chimneys part. Well, can be done with a flapper, which would dire the gases one side or the other. But that's up to you to turn, it etc.
With two chimneys, you would have one drawing more than the other, up to may be stalling point or even reversing the flow. That's no good.
For your size workshop, i think something like my rocket would do. I have 113m², with a high ceiling. No insulation, and i kept +8C°, with -17C° outside. Not too bad. Things will change this winter. As i have insulated most of the upper part which will become my pad.
Hth.
Max.