Zia-
Beyond the test scores you have, I'd suggest that you think about where your DC really shines. If it's reading, include a list of independently read books, and maybe a clip of DC reading aloud or answering questions about a book to demonstrate comprehension. If it's mathematics, send examples of problem sets with work included, and not just the answers. Whatever it is that makes you (and others!) say, "Wow!" is what you want to highlight. If there is some area you know is amazing and unusual, but don't know how to show, please let us know. I'm sure somebody around here can give you an idea.
WPPSI can have pretty low ceilings if your child took the test at age six plus. All those 17s make me wonder. On the flipside, the trouble with any test normed for very young children is that if you get a kid who is able to do math or read fluently at age 3 or 4, or even 5, their score might come out REALLY high. That's probably why DITD wants to see the 150 on this particular test. Sometimes the kid who can read Dr. Suess at five comes out looking very G, while they may be "just" bright or moderately gifted.
I think it is a good thing that DITD is looking at a number of things when evaluating prospective YSs, though it is of course a pain for the parents to assemble all the documentation. I am not a fan of the "one test" method of selection for special programming. Each piece of "evidence" you submit should give them a better understanding of who your child is and what sort of things s/he can do.
good luck!
