Well the insulation is great, I really do not think you will get a whole lot of additional savings from the exterior kits. Some of the loss in water heaters is planned. You need a certain amount of heat going up the stack to create stack action. This is the ability of the warm air to cause the flue products to rise up the chimney, high enough to reach the top of the house. If it gets too efficient, you have to install a blower motor, since the warm air just does not have enough heat in it to warm the chimney all the way to the roof and still be warm enough to reach the top. Theres a point where the air becomes the same temperature as the chimney, then it will not rise any more. But the losses from the jacket are so minimal, that you probably would not be able to calculate the savings gained. And the cost probably would not pay the additional cost of the jacket. In gas savings, you have to offset the cost of the energy saver to make it be valuable.