There are no studies that show high back versus backless. In a frontal crash when the car has head support there may be zero difference. In a side impact there may be a ton of difference, but without side impact standards and testing the only way to know is to go through accident data post crash and see who was in what. Which hasn't happened.
Generally for daily rides I'd leave a child in a high back booster until either they didn't fit, or until at least eight or nine and then they ask. Piper asked at 8.5 years if she could move to a backless in my car when I got a new car with side curtain airbags. I had no argument to say no. In my husband's car, for the next four days anyway, she rides in a high back booster (the seat expires on 11/5). I suspect she'll be going to a backless booster in there shortly. She's 10. I have no problem with that.
Wendy