We tested at 6.5 because school was not going well at all and we needed help. For the most part DS enjoyed the testing but there were times where he wasn't the most cooperative. He spent some of the time sitting until the table and throwing things while I cringed thinking about how much money we were spending a minute. In the end I will say it was worth it for us because it pointed us in the right directions (gifted/probable LD). If we weren't having major issues with school, I would have loved to hold off until he was more cooperative/mature. We ended up having to retest him a little over 2 years later to confirm his LD diagnosis since the first round wasn't conclusive. He also tested considerably more gifted the second time around which seems to match our experience so I think this is more accurate. The first round kind of gave me a false sense of - oh he's not that out there so he'll be ok in a regular class with some enrichment... that didn't work out so well.
With DD we waited until things went off the rails at school and until she was 7.5 in the hopes of avoiding two rounds of testing. She was also much more cooperative/mature than DS was at that age but even she needed reminders to refocus and stay on task. I do think we got reasonably accurate scores for her though.
It can be challenging to get accurate results for young kids. I would also check the potential schools to see if they will accept outside WPPSI testing. We have some schools here that only accept WISC (starts at age 6) as well as some that only accept results less than 2 years old for example. If the school will pay for retesting that might be fine but if you have to pay for two rounds it adds up. I would also recommend checking what the schools use to test - some of the group screening style ones aren't always the most accurate especially for 2E or HG+ kiddos.
I would also recommend taking a deep look at what these schools mean by "gifted" programs. There are huge varieties out there. Some take the top 10%, 5%, 2% or 0.4%. There is a big difference between those ranges. Some gifted programs are also more geared towards the high achieving gifted kid which is great if that is what you have but not so great if you have a 2E highly asynchronous PG kid on your hands for example.
