Yeah, distance is very critical, drops off exponentially. Lets take the portrait done by Witness as example, this is not your "typical fill flash needed coz you are in the shade" kind of photo, but a silhouette against a hazy sky at 5-6pm". If i am not wrong, looking at it, the sun is being blocked by the model's hand....just look at the brightest halo part of the sky.
I think for this, you will need a very very powerful flash gun. In the example, say taking a focal length of 35mm for full length + some sky above the head, it'd mean that a typical model is standing at least 6 metres away. It also depends on how fast your cam can sync with your external flash (ie sync-speed). Most cams can only sync like 1/125-1/250 max, best like D70 can do 1/500 (D2H can FP sync up to 1/8000 but flash power is very very much reduced). The amt of ambient light let it will still be quite a lot. If i do not remember wrongly, at 1/500s, if i meter at this kind of clouds (translucent thin type), it is way way past f22 at ISO200 at 5+. Even reflected light by white clouds is pretty bright. I am basing my calculations based on shooting during near dusk periods. Hence, my concern on flash....f22 is an awful small aperture to flash. I guess you can always try out to confirm things.
Would i shoot this kind of shot? I guess so, the model is illuminated and there is a halo up around her hands, the row of shophouses still silhouetted.
PS. GN specs has got nothing to do with this....so-called balancing of ambient light and reflected light off your subject. Anyway, to be accurate when listing flash specs, you gotta list the ISO, metres or feet, and at which focal length too. Eg, GN 38metres at ISO 200, 35mm.