I think the kiddy one with the shield goes super tall... the one without... not so much.
I think the parkway and frontier are still the tallest.
I think (don't quote me) if I remember from carseat blog that the orbit wasn't measured, but the peg has a top height of 21" for torso... so figure about the same as the probooster, so not quite as tall as the parkway, but nice.
I don't know if either of these can go backless (which with the latch features, I HOPE they do, give us more choice of latchable backless boosters... plus, the price is not low on the peg and so I would hope it would transition)
I just sent an email with some questions to peg perego about backless use and the rules about the headrest, so maybe I'll get an answer. (I'd love it if they sold it as a backless booster... it would be so cool to have another latchable backless on the market)