A higher flow rate reduces delta T, increasing output. The odds are slim that the tank would ever meet boiler temp, there will always be a bit of a differential. More water will not increase efficiency of the stove, only create a buffer for heat draws. As for the solar ordeal, I'm not sure I understand your theory. Do you plan to have 3 internal coils in this tank for boiler, solar, and domestic? I feel like an indirect water heater would be a much better fit if the only objective here is for heating domestic water. Again, 500 gallons will not help efficiency of the stove by any noticeable amount. Unless your heat load is overrunning the boiler, the added water will do nothing for you