Sounds like a major project. If your underground lines were really that bad figure the thermoplex will save you between 25%-50% of the wood you burned last year. I don't see where adding water storage is going to help you that much. You are only going to gain I would guess 50 gallons which isn't going to help your problem. As far as redesigning the flue configuration, the more cotact the flue gasses have with a water jacket the more heat transfer you will have, the more effecient your stove will be. Goggle "aqua stack". I have one in mine, I notice a difference when it gets really cold and boiler is working harder, it uses less wood during those times than it did, very little difference during mild weather when the boiler is idleing much of the time. If I had to put a number on it, maybe 5-10% reduction in wood consumption overall. Not trying to be a smart a## here but if I was you I would just try the underground lines first and see how much better it performs. If you are still not happy, start from scratch and build a boiler similar to a p&m ml series then try to sell your mahoning. It will be worth more money in stock configuration rather than a chopped up custom build. Good luck!