In most cases, I am not a fan of fill lighting "IF" GI can do the job properly. TGTP's GI seems to work quite well. Of course using fill-lights for additional effect is a situational thing. I know Oshyan pioneered some good fill light methods.
Were the GI settings on at all for this image? I thought that they were though the way you recently made it sound, they were not.
If not, try GI of 2, and turn on GI surface detail. From all the tests I've run, anything higher than GI of 3 is barely even noticeable, and above 4, UN-noticeable, except for massively increased render times.
Also, a suggestion is to play with the shadowing. I always turn soft shadows on with a diameter of 20 @ 9 samples (unless there is a prominent flat surface - the samples need to go up in such a case). When I turn soft shadows on, I bump up the sun strength to 5.0, else the image is a little too dark.
As stated, this image is incredible as-is, and these are only suggestions that may or may not make it even better. Hopefully I don't sound like I am hijacking the image here, but I really think its great!