if she is over 45 lbs, she needs to use the car seat belt, not a 5 point restraint with a booster seat. Last year, we had a long discussion with an EMT who was car seat trained. He talked about the kids he has pulled out of wrecks. He said a traditional seatbelt used with a booster seat,as a seatbelt positioner (get the one that lines the seatbelt up over their shoulder and onto their hips.... the britax and graco with the back do this well), is the safest option for kids 45 lbs and over. Then, keep them in their booster seat until they can sit comfortably on the back seat (this varies in all cars and in different seating positions in the car)...
What does this look like?
This is critical. Their butt must be against the back of the seat and their knees must bend and feet touch the floor. If they slouch to do this, the seatbelt will move up over their abdomen and injure internal organs in a crash. The belt should come across their shoulders between their shoulder and neck, not across neck or outside shoulder. This will also cause injury if it's not right. In Massachusetts the law is they must be in booster seats until they are 8 years old, In RI it's seven, BUT you need to judge the size of your child and the fit of their seatbelt in each of your vehicles. (ie, the third row of my pilot has much smaller seats so is safer w/o booster seats much earlier than the second row of my pilot, my husband's subaru has bucket seats in the back and takes much longer for them to fit into correctly).
Also, we were in a rollover accident three years ago, my 6 year old was sitting in a booster seat behind the driver and was totally fine coming out of the accident (a few cuts, not even stitches), my 3 year old was in her britax car seat and the plastic on the seat failed and cracked allowing the seat to move. Despite this, she was also fine, b/c the structure of the seat (and it's internal harness) held her in place and kept the roof of the car from crushing her. My point, carseats and booster seats save lives. This was a very serious accident in which I (As the passenger in the suicide front seat)had to be cut from the vehicle. They are fine and the booster/car seats are why. I know what you mean about wanting to keep them in internal harnesses as long as possible, but as long as they are not wiggly kids who refuse to sit still and unbuckle their belts mid trip.... the seat belt with a booster is safer.