Someone who know more will be along in a minute, but I'll have a go. IQs coming from WPPSI and from WISC are intended to be the same - they are both attempts at designing a test to measure the same quantity. WISCIV was originally capped at 160 too; it's only the "extended norms" that allow going higher (and my impression is that although this is official it might be better seen as a bit of fun than real scoring - once you get anywhere close to the ceiling, "beyond measure" is really a better description of the child's IQ than any number).
However, as you say it can make a difference if a child (especially a young one) is tired. This may be particularly the case when a child is being tested close to the age ceiling of the test, because then getting a very high score requires getting almost everything correct - even if you don't encounter actual ceiling effects on subtests (I don't know where they kick in), the test will be in effect less forgiving of slips the older the child is. If you have a subtest breakdown of your DS's score, someone may be able to give you an idea of where, if anywhere, those effects might have been showing up.
Welcome! Was there any particular reason for getting the test done, or anything else you want to talk about?
