My DS always seemed bright but not like the kids some people have on this forum.
Turns out his processing is average, but his understanding is really really really good.
But he's more like a tank than a corvette. He might take longer to process something, but he will still be able to understand things others might not be able to understand. He's not as showy as the corvettes, but he still gets the job done
When I originally had him tested in third grade, it was because he was having anxiety. Now in 5th grade, I really really see what he's able to do. I didn't see it in third grade. In fact, my first post on these boards was kind of similar- except that I was trying to understand how their scores on the tests could be so high when I didn't see much of the giftedness (other than sensory and behavior issues) in real life.
I call my DD my "sleeper" because I only tested her at the same time I tested my DS because I didn't want to shortchange her. Turns out her scores were higher than his.
Neither of my kids read early, neither are exceptional at math, DS is amazing at science and art, loves to read, and DD is over sensitive and hates reading but also likes art.
But I look back at the fact that I didn't see it with DS at her age, so I am giving her time.
So absolutely these assessments can give you a good picture of what you've got. And I'm so glad I got my son tested, and worked to get a good fit for him at school and outside of school, even though at the time I wasn't sure if I even needed to. Looking back now it has made a ton of difference. Knowledge is power