I'm not an expert (just a big rule follower

). I'd have to check the seats in our car--but know that the youngest isn't big enough for a backless booster. The older one is, but I like the back on there. She wants to go backless, but I don't want her too. Mostly b/c I can't figure out how to use that little clippy thing for the shoulder strap.
I *think* that for a backed booster--it may be 30 pounds and for backless it is 40 pounds...but don't hold me to that. I'd have to check the labels.
As long as you are following the standards..it is "okay" but I don't know if it is the best or not. 5-point better than booster w/back..booster w/back safer than backless..which is safer than just the seat...
But at what point you upgrade? I don't know what is safest..but at any point--there will be data that a child has died despite the belief they should be alive. It is all about whether the seat is properly installed, if the child is properly buckled, how the accident took place, if the vehicle rolled--the number of times it rolled.
I had heard a statistc that up to 70% or so (could be higher) of parents do not have their car seats installed correctly. I shudder whenever I watch "welcome home baby"--I have seen so many improperly stalled car seats..and they never show if they fixed it or not--just show the family driving off. Makes me wish they had a car seat expert on staff to help the parents instead of video taping the newborn in a very risky situation if the parents got into an accident on the way home.
Neither of my kids were upgraded to the next seat until well after they were big enough. Both are upgraded now. The only extra thing we do is to pull the harness out all the way and retract it after it is fastened so that it is locked. We were told by a safety specialist to do this, but noone else we know does this and I cannot find a reference in print anywhere.