Well, I guess I would start by asking what you're hoping to get out of the testing.
Scores you could use to advocate for her in her new school?
A baseline score you could use to measure how much she's learning in the new environment?
Admission to SET or some other program?
Taking the SAT at 10 would help with all of those, I'd think.
As for a 'cohort', I'm assuming you don't care about comparing her scores with those of other 10-year-olds -- since she sounds as if she's been working several grades above her agemates, you're probably used to comparing by grade level (correct me if I'm wrong). So you could just crunch the SAT numbers that way - use the TIP or Belin-Blank tables to see where she stands with respect to 6th or 7th graders.
To answer your other question: ds12 took the ACT at 10 (6th grade), after 3 years of EXPLORE, because its essay is optional and it's structured like the EXPLORE. Then last year he took the SAT. I'm on the fence about it this year because I no longer feel we need annual out-of-level testing to make sure he's challenged and learning, and he'll be taking the SAT II Math this fall after he finishes precalc; but on the other hand, he could probably use the practice, especially in Critical Reading.
FWIW, several families at ds's school have their kids take both the ACT and SAT through the regional talent search every year starting in 6th. It makes for a really hectic winter, in my opinion, since that's also when most of the academic competitions happen; that's one reason I went the independent route and had ds test in May. (I believe now our region's talent search offers more testing dates than they used to, though.)
Sorry for rambling there at the end! Hope this helps -
