I don't know if I'm right or wrong... but imo, there's a huge difference between a single individual in a seatbelt vs a 2 part combo..
Child hitting seatbelt & Booster seat hitting it.... plus the pressure of the seat against the child who is against the belt. I also wonder if there's a difference between a hard frame & a human body for how the buckle area is fits around the area.
A booster's frame is top heavy in different areas than a human body. An adult can barely handle whiplash, but what about the booster whipping forward against the child? I just don't know, not an engineer or physics major.
But I'd prefer having a lighter booster or a completely top tethered/lower anchor booster if it's over a certain weight. Recaro insists on TT with its Start -- so why not other brands? The nautilus isn't that far off on weight and bulk.
Anyway, jmho and I hope I'm making some sort of coherent sense.