There's a tad too much noise in the shadows. Were the shadows brought back in post?
Also, I've just stumbled upon a very appropriate article which should give you some insight on balancing ambient outdoor light with flash by none other than Mr Strobist himself:
http://strobist.blogspot.com/2011/12/soccer-through-sunset.html#more
In a nutshell, he's using a ND filter not to kill the ambient but to reduce the depth of field. As a result, he needs lots of power which is why he is using a beauty dish something like 2 feet away from his subject.
Here is another article of his regarding ND Filters:
http://strobist.blogspot.com/2010/06...-of-field.html
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