Much of what I am going to say is going to agree with st pauli girl. I think that you are on the right track. The CoGAT misses some gifted kiddos, but correctly identifies others. The CogAT actually offered a decent preview of DYS DD9's relative strengths and weaknesses on the WISC. She also scored in the 99th percentile, so it did not miss her.
I would also recommend the EXPLORE, as DD had a great experience with it.
The best school for a kiddo like this is a school that is flexible, IMO. These kids grow and change so quickly!
Like others before us, we had DD take the EXPLORE to see how she would do. That led to having her take the WISC. That led to us deciding to have DS6 tested so we wouldn't be as clueless with our second child as we had been with our first. DS is now a DYS, too.
How helpful is the outside testing in terms of working with your school? Well, it depends - we are still working with ours, but it did get the conversation started.
In terms of age of testing, my reading on this has stumbled across a variety of opinions. Most of the testing concerns with children over 8 has actually been with girls - apparently, they sometimes start to "hide" their gifts at this age (this was part of the reason we opted to have DD tested at age 8). I did not see any similar concerns expressed for boys.
Best wishes to you and your DC!
