I am a tech (this is my job

).
A booster seat is outgrown when *one* of the following two are met...
1. Weight limit exceeded.
2. Tips of the ears go above the top of the shell.
At your daughter's age, she is probably fine in the booster... as long as she is sitting correctly and not leaning out of position, she will be fine.
A 5pt harness *is* safer, but the booster option *at her age* is safe as well.
Check the back of the booster and make sure there isn't a height adjustment... most boosters should go taller than this to accommodate your daughter's height.
If your booster for some reason does not adjust higher, then I would consider getting a different high back booster. High back boosters also offer side impact protection in a collision and help car sleepers stay positioned properly.
A few of my favourites include the Britax Monarch, Sunshine Kids Monterey, Graco Airbooster (for kids 6+), and the Recaro Vivo.
The only thing you need to watch out for if you choose to go backless is that your daughter will *still* have head protection in the vehicles she rides in. The tips of the ears rule applies to *everyone* that rides in a vehicle... we *all* need head protection.
