The five step test is the minimum. It's like turning a child forward facing at 1, putting into a booster at four, then moving them out the minute they possibly can. Their bones need time to gain adult strength, so the adult seatbelt will work properly.
Here's the 10 year old dummy (obviously a European test) in nothing but the seatbelt.
[ame="http://www.youtube.com/watch?v=1yUl4dCK1GA"]Adult belt is not enough - YouTube[/ame]
And the 10 and six year old dummies.
[ame="http://www.youtube.com/watch?v=ntPksBGDgFI&feature=related"]maxi-cosi crash-test: adult belt is not enough - YouTube[/ame]
All of the dummies submarine out from under the belt. They don't have mature hip bones to hold the lap belt down. Their bones aren't adult strength. So you're looking at internal injuries from the belt riding up, and potentially spinal injuries from the body moving over the belt, or hip injuries from the legs impacting the front seats, etc.
We have an Audi A4, and my daughter found the Harmony LiteRider worked best for her to buckle it in. It's a very lightweight booster, so while it is a projectile, it's less of a concern than say her Turbo. Walmart.com lists its weight at 2.55 pounds. Amazon says the Turbo's shipping weight is 5.6 pounds. So the LiteRider in a 30 mph collision is 75 pounds of force.
Another option, instead of buckling it in, is to have her put it in the footwell every time she gets out. Then it'll be compartmentalized.
Wendy