I have something similar in the
Mountainsmith Day Pack which has only about half the volume. I figured I could have used it on last year's CI (I was staying in hostales so no need for bedding or towels - my
Altus could have been slung underneath) but found that, fully loaded, it tended to "peel away" from my back. I could have used the supplied shoulder strap diagonally to hold it in or the optional "strapettes" in effect giving me a 13 litre rucksack (I've used the pack+strapettes successfully on overnighters) but decided, in the end, to got for a conventional 40l pack.
The pack and strapettes ends up like this:
View attachment 56048
There is somebody who is/was on the Forum who ultralites - down to a 12l lumbar pack - successfully. He's got his own web-site dedicated to it too. I'll delve and come back for an edit if I find it.
PS - you're right, that
is expensive for what it is.