Based on our experience, a very bright 8 year old doing the WISC IV may well find they've hit the ceilings on several of the subtests. WIth a very even profile it may not make a lot of overall difference to the final score. But if he has lows that aren't offset by the true height of the highs, the overall IQ score will be artificially lowered.
This means you're not getting a full picture of his abilities (and corresponding needs) and I've found it is almost impossible to explain to officialdom what the score means.
As a 6 year old, the liklihood of hitting ceilings is lower.
