My B800's generally struggle to really overpower. Generally, in midday sunlight, I can fill any shadows cast from the sun straight overhead with a modifier like an umbrella/softbox and, depending on the situation/light, I can usually darken up the sky/ambient by 1 to 1.5 stops if I'm in pretty close. Really though, if you want to use modifiers and not have the light in the face of your subject, it's worth it to go for a B1600/Einstein to be able to knock down the ambient by 2 stops or a little more...You'll still have a tough time blacking everything else out, even with 640 w/s... Never tried my ABR overpowering the sun, but I would guess it'd be the same as the B800 (same w/s)