There's not much evidence either way, and there probably won't be, because the problem is a child over 4 in a high back booster is already very safe. You can't improve a huge amount on it and the amount of children travelling in rear facing seats (where you might well see an improvement) at that age who end up involved in an accident is so miniscule it is negligible and can't give you any real life statistics. You can only go by crash tests and that is not the full story.
There is some good data now showing that age 4
The idea that FF in a harness rather than a seatbelt is drastically dangerous to the spine/neck turned out to be something one of the companies marketing impact shield seats put out there in order to bolster support for their impact shields - apparently. In real world crashes impact shields come out about the same with 5 point harness forward facing, because while they do alleviate some of the neck loading they can put too much pressure on the abdomen and they can eject the child especially in a rollover, an offset crash and/or a very young or slim child. As said overall it comes out about the same as a 5 point harness.
Australia is probably the place to look at - they have very little rear facing, they have had 5 (in fact 6) point harnesses with top tether for a long time, and they have had safety campaigns focused on keeping kids harnessed up to the age of 6 or 8 even so there is more data to compare older (and even younger) FF children.
Rear facing is definitely safest but the idea that FF in a 5 point harness is terribly unsafe is not really evidence based at all. The risk of internal decapitation is horrible and I won't tell you it's zero, but it's not as common as some of the FB groups would have you believe. It's a pretty rare injury - one US trauma/children's hospital reported less than one case a year. Some people say that it's rare because children who die in car accidents don't always get this injury recorded, but the numbers of children who die in car accidents in the UK are also incredibly low, especially when you screen out the number of children who were not in a seat at all. And our national average time for turning forward facing is about 2 years old (which I already think is a brilliant improvement over 5-10 years ago!!)
RF is worth doing but it's not worth stressing out over. If it's working brilliant - keep doing it. OTOH if it's causing problems and your child is not a tiny baby, the trade off is likely worth it.