My thought on testing now or waiting mostly concerns the function of the testing at this time, not the ceiling. The school district appears to be expediting access to GT services (by two years!) contingent on testing him now. Waiting six months on the expectation of a more robust score on the WISC also means waiting over six months (which takes you to around May of next year--the end of the school year) before accessing GT services. In the intervening time, there is a risk that his first formal school experience becomes irretrievably tainted with negative emotions, due to the lack of access to offered services.
And FWIW, if he's a kid for which ceiling effects are a real concern, then there is a good chance that he will be in that 145+ range anyway. Also, nothing stops you from having him re-tested after his sixth birthday. It's not the same test, so although I would prefer to wait at least a year or two in-between, it's not technically an invalidating retest.
