I would guess there's a very high correlation between percentile achievement in 7th grade and percentile achievement as a Jr/Sr. Raw scores will certainly be higher, but scores at a 90th percentile in 7th would likely indicate a performance at the 90th percentile later. But I haven't seen formal documentation of this.
Part of the challenge is that those taking the SAT in 7th aren't a representative sample as 11th/12th would be - it tends to be kids from gifted programs, so skews higher. I think TIP will publish a percentile analysis, as does JH CTY, but that's only among the test takers.
There's an attempt to statistically project the numbers to the broader audience here -
7th Grade ACT/SAT scores . But I'm not convinced this is that accurate - my DD12 would be 99.999+ percentile given the mean/SD proposed here, and I don't think that's right.
I also find this one interesting -
SAT gifted long term study Regarding the specific question - I don't think the scores actually do anything, other than define eligibility for TIP/CTY etc. courses. They may serve as an indicator for someone not in a gifted program that further evaluation is warranted.