Unfortunately, no! Of those of us who dork out on technical matters, I would guess this rank #1 of a study we'd love to see. But we're all out of luck, as no such study exists.
What I'm guessing you've seen referenced is a *theory* on neck loading in older kids. It boils down to this: in young kids, say ages 1-3, we have the problem of the spinal column not yet ossifying. So when FF in a crash, the spine can stretch/break, hence we RF as long as possible. The Swedes pioneered this ERF business, and even if we don't practice it, there are studies that allow us to acknowledge that it exists.
The spine ossifies between ages 4 & 6. So in the "ideal" model, we turn FF around this time, but the question is -- to harness, or to booster? The Swedes turn their kids directly into boosters, even their sqirmy barely-4-year-olds, and they say it's safer to booster those kids than to subject them to the neck loads inherent in FFing harnessed seats. The school of thought here in the US (and I'm inferring here, as they've never made any direct statement,) that appears to be supported by NHTSA, is that there are benefits to harnessing that are outweighed by the potential neck loading.
Theoretical pros to boostering in this age group: allows head, neck and spine to move as one unit
Theoretical pros to harnessing in this age group: keeps the child in position without opportunity to wiggle, protects better in side impact crashes
So why are there no studies? My understanding is that the simple fact of the matter is that there just aren't enough data points to do a reliable study. It's the same reason the only domestic ERF study only includes kids under 2 -- there just aren't enough kids RFing past age 2 to do a reliable comparison. We have foreign studies to prove that data point, but EH isn't common enough in other countries to do a study there, either.
This is also an issue that tends to "swing" a lot -- back when Kyle David Miller was big news, it was very much a tone of "OMGZZZZ harness as long as possible!!!" That sentiment, elsewhere, is actually what brought me to c-so. But by the time I got here, things had swung back to, "Well, we don't know which is safer, so take your pick once your kid is 4.5-ish." With NHTSA's statement, things are swinging back a bit the other way.
I am probably pretty far up the "worry" spectrum about FF harnessing and neckloads -- in my ideal world my kids will RF until they're booster mature to avoid the issue entirely
-- but even at that, it doesn't wig me out all that much. If someone wanted to harness their 8yo because it's easier to buckle that way, meh. I worry a lot more about the impulsive 48lb 3y9m child I had at a check this week going into a booster, or the 25lb2yo turning FF.
It's hard, because we know certain things -- we KNOW kids are safer RF until 4, we KNOW kids are safer in a booster than a seatbelt between the ages for 4 & 7. We know kids who transition to a booster or seatbelt early are at more risk. But we don't KNOW whether kids are safer harnessing as long as possible, or moving to a booster at a reasonably mature age. There's not a lot of data from which to form your own opinion, either, but all we can do as parents and caregivers is look at the information that we have available and form the best opinion that we can.