There was research done in Australian on younger subjects comparing the SB5 and the WPPSI which indicated that the two tests were fairly comparable but for a certain percentage of children their scores would be quite significantly different and there was no apparent pattern, there were kids going high or low on both tests and they checked for which order they were given and various other possible causes (ie the kids weren't doing much better on the first test or the second test, etc). In the end the conclusion was that the effect was due either to one or both of 1) small kids have variable performance 2) some kids may simply be better suited to one test or the other.
What I personally have seen is that my girls who have both done at least one Wechsler test and the SB5, their verbal scores were almost identical across all tests used (no more than 2-3 points variability) but both my girls do far better on the non verbal of SB5, half a standard deviation or more better. There is something about the WISC non verbal that doesn't work for them (and it's not the block design, it's the cartoon based tests).
Also being tested by a gifted expert helps, particularly with my more gifted child the gifted specialist did a far better job of drawing her out and of moving at her pace.
