You'll struggle at mid day, my profoto acute B2 600r is 600ws so about twice as powerful and it is marginal with a big modifier - 1000ws would be better. It's all about distance and modifier though, if you get a silver plm you'll lose maybe 1/3 of a stop vs maybe 2 stops from your softlighter so choose wisely. If you get a super efficient tight reflector you'll be fine but whether you like the light or not is another matter!
Kitehog, your example does not look to be at mid day - the sun is pretty low.
As others have noted it depends on the subject distance & modifier used .. if I were shooting in noon sun a lot I'd go with the 1600, the price difference is not that great and you will most likely need the additional power at some point.
Have only shot it in open shade, but the AcuteB with its 600J should manage broad daylight in a useful way. As noted by dmward, Sunny 16 allows f/11 @ ISO 100 and X-sync of 1/200. AcuteB w/ Magnum @ 2 meters yields f/64 according to Profoto. That allows one to knock down ambient sunlight by two stops and still have two stops in reserve. My 1Ds @ ISO 50 actually has three stops in reserve - enough for a medium softbox, methinks. Need to experiment...
you can balance things out with a B400 if you're pretty close... with that said i just added a B1600 to my setup that started with a pair of B400's... i've also painted my silver PCB 22" HO beauty dish krylon matte white...
