If you have a 6yr old then you can have him take WISC-IV. You only need 145 score on WISC and DYS also considers the GIA index which doesn't include the processing speed. If your child hit 145 on WPPSI chances are quite good he will reach 145 on WISC too.
As for WPPSI and SBV we did try this (expensive) experiment. DS5 hit 99.9% on both PIQ and FSIQ on WPPSI but he was short of the 150 cut. His VIQ too was in the 140's.
He took SBV a few months later. His non verbal IQ on SBV was the same as his PIQ on WPPSI. His verbal IQ took quite a hit but I wouldn't blame the SBV for it as he still pulled two 19 in the verbal section. The verbal section comes at the end of the test and to us it looks like by then he was done with the test and put hardly any effort in it. His oral language achievement test results are much more aligned with his WPPSI VIQ than the one from SBV.
HTH. Feel free to PM me.
