Hmmm. I have issues with the WISC-IV. My son didn't score as gifted and the psychologist couldn't even calculate his IQ, yet he was ahead in a few areas as well. We haven't done a SB for him (I'm kind of disenchanted by the testing process for kids, to be honest. I wasn't tested until adulthood, and frankly I think there's more accuracy then). Also I've heard that the WISC is only good to about IQ 130 and anything higher is better served with the SB.
Enh. I think they each have their strengths and flaws. I do prefer the structure of the WISC-IV to the SBV, though, as it has a more coherent theoretical basis. From the sound of it, I'm guessing that your son may have had a highly skewed profile, which means that the FSIQ would not have been the best representation of his ability (probably the VCI or PRI would have been better), which would likely be why the psych didn't calculate an FSIQ. This does not mean that the instrument does not provide good information on giftedness; with a 2e or other low-incidence profile, there probably isn't an instrument out there that would really capture the full potential.
For HG+ individuals without 2e, testing in middle-to-late childhood is actually best (8-12 yo), as there is still enough range in the tests to be able to use extended norms. Scores may be more stable in adulthood, but ceiling effects become more problematic for the highest-scoring individuals. (Not an issue if we're looking at <145.)
