SB24 is pretty powerful. If you're shooting 400 ASA, you'll likely have enough power. Doubtful that it'll be powerful enough to supplement/overpower direct sunlight.

What camera are you using? If you're shooting MF, I hope you have some Leaf Shutter lenses... The slower sync speed of FP shutters on MF doesn't work too well with outdoor flash. My ETRS is a blast to use with flash though... 1/500 sync on every lens.