that booster is deceptively wide... I remember when we got it, Damian already passed the 5-step test, but he fit in that booster, while he'd gotten too wide a couple years earlier for the Turbo...
anyway, I'd definitely use it... there's one thing we all agree Graco can do... make a good booster.
When Ruthie was about to outgrow the Nautilus, I actually considered putting her in Jeffrey's Monterey and him in the Backless Nauti.. it's so nice and thin in the back, it offers amazing grow room as a backless.
But she didn't fit the Monterey well enough for me... so then I considered using it as a booster... but she doesn't sleep well in it, AND I remembered my niece could use it (especially now that she's pg with baby #3... she'll be able to use the infant seat I bought her when she had her first, as her oldest is only 3yrs old, then the scenera for the new baby... and her two girls will still have safe seats... the bolero I gave her and now the Nautilus (I wish she'd rearfaced them longer, but since she had made her choice, I passed down seats I knew she'd easily use properly))
Anyway... had it not been for poor fit in the Monterey and my niece having such a use for a harnessed seat, I would have used it as a booster if Ruthie slept comfortably in it (she's such a head flopper that in booster mode, she'd not stay up straight (I tested her in it at night))... she sleeps perfectly in her Vivo.