I'd start by approaching the school psych and basically state what you did above - "I've been researching both the Weschler and WISC IV and it is my understanding that both tests have timed components. We believe she has potential issues with processing speed and/or working memory and we're concerned those issues will impact the overall score. My understanding with the WISC IV is that a GAI is a possibility if that is the case, is there an equivalent for the Weschler?" or maybe just go with a simple "if this was your kid, what test would you want her to take and why?".
