I would say it depends on a few factors Peter. First, is your exhaust temps high enough to extract more heat and so that you don't loose too much draft and have potential smoke-back issues, hard cold starts etc.?
My elevation gain was made from mass - Yours will be bare metal which will obviously transfer the heat much quicker, so going back to my first question of current exhaust temps and how much they can be lowered will affect whether mass or metal should be used, if at all.
Matt Walker recently said in a stove chat episode that roughly 200F was a good exhaust temp for an effective all season draft whereas 300F was giving up too much. This number is the centre of the exhaust stream, not surface temps. I've even had it lower than this with no draft problems but of course it really depends a lot on so many factors like wind direction, temperature differential, climate, humidity, length of vertical exhaust pipe etc...etc... Each situation is totally unique.
Also, if you did go with the half barrel idea and weld them together, would you still be able to lift it off for cleaning/inspection?..... will Mimi be able to help you out?
However, if you put the half barrel on the bottom and remove its top, you could use a clamp to secure them together (assuming they are compatible) and readjust the seal at the bottom to accommodate a rimless base. This would solve the weight issue.
Note: Having 1 1/2 barrels will mean that that it will direct the instant radiant heat higher up which won't be as noticeable at body height if this is not a problem.
All in all, its a simple modification and relatively easy to switch back if it doesn't work out for you so I don't think you can go wrong either way...after all your a scientist right?