Also keep in mind 'statistical significance'.
Take a sample of 1000 kids, 6 years old, 50 pounds (THIS IS NOT A REAL STUDY, I AM MAKING UP THESE NUMBERS TO SHOW WHAT I MEAN).
They are all in identical accidents. 500 of them are in higher weight harnessing seats. 500 of them are in boosters.
Of the harnessed children, 21 sustain bruising. Of the boostered children, 23 sustained bruising.
Were more boostered children injured? Sure. Is it significantly significant to the point that parents should be running to purchase a hwh seat? Absolutely not.
Older kids have more mature bodies. Larger bodies means that energy is transferred over a larger area instead of directly concentrated on a small area. So, a 30 pound forward facing child in an accident is over all at a lot more risk of injury than a 60 pound child.
Here's another example. Take two four year olds. One is buckled into a correctly installed harnessed car seat using a top tether. The other one is buckled into a car seat, is wearing a crash helmet and has a fire-repellent suit on.
Are they both safe? Yes. Is the kid in the helmet and suit safer? Arguably. Statistically, is that kid less likely to get injured because of his added gear? Eh, probably not.
My almost five year old is in a harnessed seat. He can sit maturely in a booster (I started booster training him when he turned four.) He probably has another 12-18 months left in his harnessed seat and I intend to leave him there because why muck with a good thing? If he outgrew it on his fifth birthday, though, I would buy a booster, because he's big enough, old enough, and can sit properly.