Ah, long torsos. Sigh. My long-torsoed child outgrew the models with the headwings (because you can use it with shoulders over the top slots if the child's ears are below the top of the seat and the child is comfortable- but the headwings block such use) between 45 and 50 lbs. but she is heavier for her size (all muscle) than your kids. Um... Yeah. The model without the headwings she still barely fits in but at 2 inches over the top slot she's getting uncomfortable and so I'm not using it for her any more. (She weighs about 53 lbs.) A more typical range is 60-70 lbs. to outgrow the harness by height- but with long-torsoed kids, that goes out the window.
Many kids who are skinny and long-torsoed, like yours, are never going to be able to use the booster mode because it has a 50 lbs. minimum. So their shoulders will go over the belt guides, rendering it outgrown, before they hit that minimum.
I would not plan on it being the last seat except a backless booster you ever need. Not with a long-torsoed child. At the very least you will want a high-backed booster which provide better support, especially if your child outgrows it by height around age 5... If before that, you'll need a taller harnessed seat.
It is a fantastic seat for rear-facing a long time. It's great for plane travel with kids over 2 who can forward-face (and sometimes you can rear-face it if you have an angle adjuster, on some airlines.) It will last a year to a few years past outgrowing it rear-facing forward-facing. There is no guarantee that you'll never need anything but a backless booster after that, though, so I just don't want you to get a false hope of that if you have a long-torsoed kiddo (a pain I know all too well, lol.)
And yes, children need to pass the 5-step test before they use the seatbelt alone (though honestly the belt needs to be positioned correctly IN the booster and they need to be able to sit properly the whole trip in it before they move to a booster from a harness.)
