My DS6 took the WJ-III at 5 years 1 month. The writing parts were very hard for him especially the sentence writing (he was actually in tears at one point). The tester said that he immediately knew the answers but was painfully slow at the writing due to his perfectionism. He also got his lowest score on the math fluency. Again--knew the answers without having to even think but each number took him so long to write since he was still making most of his numbers backwards last year. Since he started school this year his writing has become a little bit better and easier for him to do. Despite his extra time on the writing sections his scores were still in the GT range.
I have also been told the WJ-III does have higher ceiling than the WIAT.
Good luck.
