I had a mountain bike with 22x28, which is ~20". At these gear ratios you can get lower more easily by swapping to a MTB crank and taking a few teeth off the chainrings rather than adding several to the freewheel. At a certain point, exhaustion comes from total energy expenditure climbing a mountain, not wattage output. Taking aerodynamics out of the equation, which I assume is a non issue at this gear ratio, it is still the same work done if you go slow or go fast, and you've burned the same amount of calories.
Rather than just having a really low bail out gear I prefer multiple gears so I can get a good balance of efficiency and speed. There has been times where I felt geared too low wit a bail out gear, and was more tired at the top than if I could have gotten a gear where I could get a good rhythm.