As with other round head flashes, the FJ80 does not report a guide number in its specs. This is due to the fact that a round head does not deliver its light in a fashion similar to standard speed lights. Most standard speed lights will report a guide number for it's maximum zoom (typically 105-200mm). The resulting guide number does not relate directly to output but to output based on the flash head's beam angle or zoom and the rectangular shape of the light.
A typical speed light will be focused by a Fresnel front into a rectangular frame meant to mimic the shape of the sensor or film plane. Using guide numbers for standard speed lights can give you an idea of the power and efficiency when comparing one standard speed light to another. A round head flash like the FJ80 while utilizing 80Ws (compared to the typical approximately 65Ws of the top of the line OEM speed lights) spreads that light more evenly and over a round area. This makes it great for use in round modifiers and for emulating a more smooth natural distribution of light as would occur in most real world light situations. So while guide numbers may be useful in comparing standard speed lights, guide numbers do not offer an apples to apples comparison to a round head.
Put simply, in addition to better light quality, the round heads higher Watt Seconds will typically record greater output on the subject than a standard speed light when matched with a round or octagonal softbox. Modifying the round head with a long throw reflector may also deliver a greater amount of light over distance as well. The longer Fresnel focusing range and rectangular light produced by standard speed lights offer more efficient delivery of light to a specific (rectangular) area and may be a better choice where maximum output at greater distance with no additional modification is required.