The power output of the water heaters is the crucial point. The "high performance" heater is 76 kBTU/Hr, while most water heaters only are around 30-40 kBTU/hour. This larger burner allowed the smaller tank to be able to "keep up with the demand" for quite a while. Another reason is because it maybe would be configured to heat to a much higher temperature, and then use the mixing valve that in includes to reduce the temperature to the reasonable level. I'd consider this "cheating" if it is what they are doing, though it is allowed by the government.
I'm guessing that the listing says one person because it is a small tank, and not based on the FHR (which they should have paid more attention to).
I don't think that you are missing anything based on the specs on FHR. But, you do need to pay attention to the size of your natural gas line. You'll need a higher flow rate with this model since it's usage rate is much higher. You also might need to provide more "leaks" in your house (and room containing the water heater) to allow for it to get sufficient air for combustion. This particular model uses air from its surroundings, instead of having a pipe to bring air in from the outside. Insufficient fresh air will generate CO (very bad for living things) and will decrease the water heater's efficiency. Make sure that you have a CO detector installed near the unit, whatever your choice ends up being.
The first hour rate is an odd metric. It measures the number of gallons that can be supplied at above 110 F, with an initial temperature of 135 F. This is very difficult to use in calculations.
I'd approach the calculations as an energy conservation problem. Use the recovery rate to calculate the burner output (1080 kJ/min in this case). Then (ignoring that entropy is gained in the mixing valve and the heat capacity and density of water depends on temperature), decide on what temperature shower you want, and what temperature water you want in the tank. Use the heat capacity of water to calculate the amount of energy stored in the water heater, the energy usage rate (power out), and the burner energy rate (power in). Assuming use of 6 gpm, input water at 7 C, the tank at 75 C, and using water at 38 C, I calculate that you would have about eight minutes of water. With the 55 gallon tank, you'd have about 18 minutes using these assumptions.
Just by looking at the 90F recovery rate, you get about 80 gallons/hour. So, you could use 80/60=1.33 gpm indefinitely (with a rise of 90 F). But, you probably only need a rise from 45 to 100 F for your shower. In this case (55 F rise), you could use about 2.2 gpm indefinitely with this particular burner.
At least where I live, the supply charge for the first therm is roughly 20x the supply costs for the next therm. I could be wrong, but I don't think that this will save you much money.
Best Answer
If you're only worried about the water heater, then you can turn it off and drain it.
Shut off the gas
You should see a gas pipe connected to the unit. It will likely either be rigid black pipe, or flexible yellow pipe. Follow that pipe until you find the shutoff valve. It should be a quarter-turn ball valve, so you'll only have to turn it a quarter turn to shut it off.
An easy way to tell if the valve is open or closed, is to look at the handle. If the handle is in line with the pipe (parallel), then the valve is open. If the handle is across the pipe (perpendicular), then the valve is closed.
Shut off the water
Since you're only concerned about the tank, you only have to turn off the water feeding the tank. Locate the cold water supply pipe (possibly marked with a blue ring on the tank). Follow the pipe until you find a valve. In this case, it could be a ball valve or a gate valve. As with the gas valve, a ball valve will only require a quarter turn. A gate valve, however, will require multiple turns to shut.
Drain the tank
Connect one end of a garden hose to the spigot at the bottom of the tank, and run the other end to a drain that's lower than the spigot. Open the hot water valve on a few faucets in the house, to allow air into the plumbing. Open the valve on the spigot at the bottom of the tank.
CAUTION The water draining from the tank will be HOT.
If you have time, turn the tank off and allow the water to cool before draining the tank. This could take a bit, as the tank is designed to hold the heat.
When you get home
When you come back, make sure the spigot at the bottom of the tank is closed. Open the valve on the cold water supply line, and allow the tank to fill. You'll know it's full, when water starts coming out of the faucets you opened throughout the house.
NOTE: As the plumbing fills, air will come out of the open faucets. As the system nears full, water and air will spit and sputter from the faucets.
Once full, close the faucets. Turn on the gas, and follow the instructions on the tank to turn it on (light pilot, etc.).