That's a good plan. I haven't had a LOT of contact with the school gifted specialist(they seem to keep her hidden away), she has been the most kind and affirming of DDs' HG/PG educational concerns.
Also, mimi, it's hard not to take all this personally, I know from my own experience.

It's probably not you they don't trust, it's more the IQ test given by a tester they are not familiar with that they don't trust. I don't want to get into whether or not that is fair, but it's a good thing to keep in the back of your mind.
FYI, my DD6 has a lower IQ number than older DD and 6yo is currently outperforming DD9 in math compared to when DD9 was the same age. Way outperforming. Hard to tell if it's because the younger one has access to more materials than older one, we'll never know.
Regardless, your kiddos scores are way high on both aptitude and achievement. I guess you should ask yourself if you have anything to lose by having the school administer the tests. If they administer the WJ achievement test, which is out of level, it might be a real eye opener for them.
While this is your child's LIFE, this is the administrator's JOB. Totally different perspective on the thing than you have. I've had better luck when I attempt to approach conversations with a very professional tone, I seem to get further with that style. Good luck with future talks.
Neato