Airforce, i am not sure if adding a small tank will let you see much of a difference, and i am not really sure you would see a big difference with a big storage tank....however it is my belief that a big storage tank will be more efficient in the event that you would not lose the unburnt btu's going out the stack as smoke at all the start up and shut down cycles..then on the other note you may lose more btu's out the stack while your fire is roaring trying to heat all that water
then again if there is no diference and it was all equal i also think that you would be at least by having one hot fire and avoiding all those start up and shut downs when these types of stoves smoke badly (or so it is stated) that you would at least be burning your wood more "eviromentaly friendly"
i am drawing up plans now for a new stove that would use a 1200 gal tank....it may nto be in operation for 2 years but i am going to give it a go. i only have my time to invest. i got all my steel at "freebee" cost