Coming from a guitarist background - the best straps for guitar are the 3" straps. They distribute the weight across your shoulders, relieving that ache during long sessions. Thin 2" straps with padding aren't nearly as good as 3" straps without padding. Extra padding isn't nearly as important as weight distribution.
Are there UL backpacks out there that utilize wide, breathable straps? Maybe of a mesh material.
Now of course, if you are truly ultralight, weight on your shoulders is not as big of a deal. In my mind, it would still be an improvement.