Amber, I'm just bumping this for you since you've not gotten any responses. My suspicion as to why no one is chiming in is that we just don't know. It is so individual. I've seen kids do well on one IQ test and not another, well on both, etc.
I guess that, if someone were going to guess, they'd need some idea of how his scores played out on the WPPSI beyond just the FSIQ. The SB doesn't have the timed aspect of some on the Weschler tests. I'm less familiar with the WPPSI than the WISC, though, and timed may not be as big of a deal on that one. If timed aspects decreased his score, that may be a plus for the SB.
If he has a better visual short term memory than auditory short term memory, again he may be a bit better on the SB than a Weschler test or worse on the SB if his auditory memory is the better of the two.
If his WPPSI strengths leaned toward visual spatial and not so much the verbal areas, that would again support the possibility that he might do better on the SB. The SB has a reputation of being better for mathy kids b/c it doesn't have such a huge verbal section but does have quantitative and visual spatial sections. The Weschler tests, save for the adult version, do not have quantitative subtests other than an optional arithmetic section in working memory on the WISC and that isn't really a true math ability test.
Overall, this is all maybes and mights, though. You won't know until you test. He could come out with a FSIQ of 120 or 150 and neither would be surprising. Preschool IQ tests are fraught with uncertainty and I don't think that any of us can say that it is likely that a child who scored in the 99.5th percentile is likely to repeat or better that performance when retested on another test later. None of us can say with certainty that he won't, though, either.
In terms of when to test, the SB-V also has a reputation anecdotally of scoring higher the younger the child, so I might do so sooner rather than later if I really needed a super high score and was more concerned about that than anything else.
