I wouldn't write a letter to the psych - the time frame he's giving you really isn't terribly unreasonable at all, and if he's a well-respected professional, I'm guessing something unexpected came up and he'll do his best to get the report to you as quickly as possible, so all the letters in the world won't help you get it before then.
OTOH, I think I'd call and ask if you could get a list of the WISC scores - you don't need the full report, just ask for a breakdown of VCI/PRI/PSI/WM/FSIQ. Our neuropsych was able to give us that info on a sheet with date of testing etc that her computer spit out - I don't know if it's a standard WISC report or not, but we used it didn't take any time for her office to generate and we were able to use it when applying to a program that we didn't want to provide with a full neuropysch report.
Re school, why not ask the principal if you could go ahead and schedule the achievement testing while you're waiting for the WISC results? The one thing that I'd really consider is - your dd isn't going to be a different kid if you find out her FSIQ is 160 or if you find out it's 120 or if you find out it's 100. She is who she is, and something that's going on at school has led you to think she needs a larger challenge, right? So what happens if you find out she isn't "gifted"? Are you really going to think, ok, just leave everything alone as is, or are you going to still have a student who needs more challenge in school?
I think what I'd focus on right now, for advocating, is what has happened this year in school and what type of challenge do you think she needs. What other types of data do you have to show she needs something more? Work samples? Work she's done outside of school etc? Conversations you've had with her? Her teacher?
What does her teacher think? Having a teacher who thinks she needs acceleration may actually be a more productive tool when advocating than any set of WISC scores.
Best wishes,
polarbear
