The elder has many attributes associated with giftedness (super-intense, recognized numbers and letters early, highly sensitive, etc.). The younger is also very smart, very high EQ, and less intense. Both parents are gifted (likely HG/PG) so we'd always assumed that both kids would be gifted as well.
I would step back from looking at the world through the lens of IQ numbers. Each child is an individual with their own set of needs - the #s come into play when looking at program requirements, but just because a child isn't over some magical hurdle of an IQ # doesn't mean they don't need challenge in their education. I also think it's a bit misleading to assume that only extremely high-IQ children are intense, highly sensitive etc - if you've seen one extremely high-IQ kid you've seen exactly that - one child. I've worked with a lot of children, and it's absolutely possible to be super-intense or recognize #s early and not have a sky-high IQ. At the end of the day, everyone matters, no matter what their IQ

Elder has complained for years of headaches, blurriness, other vision problems; after multiple trips to optometrist, pediatrician, ophthalmologist, etc., we found a behavioral/dev optometrist who ID'ed some issues and VT starts next week, estimated to run 10-12 weeks.
This is wonderful that you discovered the developmental optometrist - VT was absolutely life-changing for one of my dds. The one note I'll throw out there though - I'm not sure the issues were huge in your ds' case, if the VT only takes 10-12 weeks. Is he going for multiple sessions per week or just one? I'm only curious because my dd and the other children I've known who've needed VT usually had programs that lasted for around 9 months - 1 year. In most cases, those first 3 months saw the largest gains, but the remaining program was also important and valuable. So I'm guessing in your ds' case the issue isn't extreme.
We had them assessed this fall (we're evaluating school options - see next question), both on the WISC-V. Elder came in at FSIQ 125/GAI 124, younger at FSIQ 129/GAI 130. We didn't expect 140s but are a little surprised that they're not solidly in the 130s.
Both of these scores are good, strong scores - whether they could be higher or not, they are great scores.
What's the likelihood that elder's vision issues affected performance/score?
Absolutely could have - you can look for clues if you have the subtest scores, as some subtests rely on vision much more than others. If you have the scores and don't mind posting them, there are folks here who can look at them and let you know if there appears to be an impact. In general, any deviation in subtest scores greater than 1.5 standard deviations is considered meaningful. Just from looking at FSIQ vs GAI, I'm not so sure about an impact from vision - both FSIQ and GAI are really close. The impact for my dd with vision issues was significant in processing speed subtests, which lowered her FSIQ relative to GAI. However... she was tested with WISC-IV, which has a different configuration of subtests, and I'm not familiar with WISC-V enough to have a clue whether or not you would be seeing a higher GAI in the event of vision issues.
Switching schools requires a teacher or principal recommendation, so is not lightly done (and school admin may take it personally); if we apply elsewhere and either don't get in or decide not to take the spot, the current school will be even harder to work with. School applications are due early January so this is something we're wrestling with right now.
Are you sure you can't apply without a teacher recommendation? Some schools will let you by with a recommendation from someone outside of the current school in a situation like this if you just tell them you're uncomfortable asking for a recommendation from current school and why.
Best wishes, and keep us posted!
polarbear