I will agree with Josh, Jerry, and Buck. As well, Dale W.'s first paragraph pretty much sums my experience. I understand the points made about super puffy bays and core temperature in extremis and will just accept they are true.
I like vests and will wear them with other layers on the arms and be happy. I am not happy with very little or nothing on my arms. If it is chilly and i have on a vest with windbreaker and t-shirt, i am more happy to add a bit to my arms rather than add more to my torso...
I think it might be worth considering variable weights between arms and torso. You want the warmest thing for the weight.. The warmest (balanced with most comfortable) might be, say, 2.5 oz/yd in the arms and 6 oz/yd in the torso. I realize you want down but weights of synthetic materials is the easiest way to make the point... I just grabbed the weights of the top of my head but they also illustrate the point.
Now.. What the balance of fill distribution should be.. Who knows? See several manufacturers use variable fill and advertise the weight of synthetic fill. This might help you to determine a ratio of torso to arm fill. You might want to cheat a bit more to the torso as i do think more to the torso versus the arms is better and the manufacturers might be a little conservative in that they realize people might not buy a jacket that seems to have very lightly insulated arms...?
I just don't think a super thick vest only is best.. If it were, why waste all the sleeping bag fill on your legs? If super puffy torso is the complete answer, take the fill in the legs and put it in your torso.. :)
PS - I didn't realize this was such an old thread! The more recent comments do seem to be in favor of the jacket. Reading the thread, i remember thinking the comments on page two were more in line with what i would recommend. Those are the new comments. Interesting the different feel in the responses between November 09 and now... Or, maybe, it is my imagination. :)