Last time I installed my Nautilus I asked my son to humor me for a minute & tested out hbb mode.
The shoulder and lap belt fit him perfectly... it was like it was made for him.
My DS 1 is 6.5yo, 40lb, and on the 3rd slot on the GN... a little guy.
I just purchased my 3rd GN, this time an Elite version, because I needed the lock-off (lock-off works beautifully by the way). It arrived today
I plan to eventually use all 3 of them as hbb & possibly lbb provided they continue to fit so well... both my boys still fit in the harness, at least for a little while longer.
From my research all boosters (combo seat or not) are hit and miss on fit... it all depends on the seat, your child, and your car. Best you can do is try them out & see what works best.