Are these standard hot water heaters or are they high efficiency? I am going to assume that they are standard. In that case the water will travel down the pipe and directly onto the burner but the burner will have a cap on it like a sealed burner on a gas stove so the water should just run down the side and not cause any damage. The water will probalby drain out of the tank and onto whatever is below the tank. Hopefully not drywall or insulation. I would be more concerned about that.

you beeter build it strong in that area, a full wh weighs a ton! well not quite a ton but it is heavy and with the small footprint exerts a large load. you will also want a pan udner it, imagine what would happen is the tank sprung a leak, gallons and gallons of water would flow befor you even knew it was leaking, this will ruin the ceiling and insulation. surely you don't mean you are going to put the main breaker panel in the attic? even though you don't have local codes restricting you, your insurance company can deny you coverage if you don't follow standard code practices, one of which is the panel must be readily accessible and visible.

You should check either with your local gas company or with your Town to find out about building codes, and to make sure you are not in violation. You probably need a new water heater. The useful life of these things is 20 years the most. They usually quit very suddenly and leave a big flood. You should consider yourself lucky that yours is "warning" you that it's time to replace it. The new gas heaters and water heaters are much more efficient than the old ones. Your gas bills will be lower. And, because of their higher efficiency, they emit less smoke. Therefore, even if your chimney is too small for your currect system, it may be fine for a new one. But you do need to get impartial assessment from either the gas company and/or your local municipality. Your system may have been approved according to codes that were used when the chimney was build years ago. But it may now be a violation because codes may have been updated. In that case, your system may be okay if there is a "grandfather" provision that avoids forcing all homeowners to update their systems.

Okay, something isn't right here. If you had a 40 gal water heater and after 9 years you couldn't get a 15 minute shower, I could buy that. But an 80 gal water heater?!? Do you have multiple shower heads or otherwise move a large volume of water? A broken dip tube or a thermostat issue could cause this problem, but if you tell me that you don't have multiple shower heads, I'm hesitant to conclude that it's sediment. I'm a big fan of gas tankless. They save about 39% on water heating costs, take up less space and don't run out of hot water. Of course they are more expensive and require proper sizing.