I like the idea of work samples. If this is your first K child at this school it would be good to have some idea of how much your child's skills differ from the most enriched curriculum they provide at that grade. While I like tests like the WJ to some extent, I do think that the age and grade equivalencies aren't a full representation of what a child needs. I always use as an example my dd13, who as a newly 7 y/o 2nd grader had grade equivalencies of 18+ in the Language Arts parts of the WJ-III. While she really needed much more than could be provided by a typical 2nd grade class and did later grade skip and subject accelerate, she didn't need post high school instruction in that area.
What type of math instruction, for instance, do you think would be appropriate for your dc? Can he comfortably do long division, multiply, etc.? I am assuming that the basics of addition and subtraction are down with those types of scores and what I remember of K did involve a lot of pattern recognition and one to one correspondence in preparation for those skills. If you can bring in both the WJ scores and some examples of what he is so comfortable in that he does not need instruction in it anymore, that might help the teacher plan for what he does need to be taught.
Likewise in reading, I'd bring both the WJ scores and a book or two that he has read independently recently such that he doesn't wind up getting a bunch of phonics instruction, for instance. Like others, though, I wouldn't walk in with a pile of this type of stuff and start off the meeting with it
! Good luck. It sounds like you've got a lot of advocacy ahead of you!
