This saves space. I need to re-run a/b tests but we were allocating 32bytes of space in each slab for bits. For awkward large sizes this can waste a whole items worth of memory.
I will post some a/b tests of keg layout later to show how much of an impact it makes. It is simple enough that it should go in regardless.
The boot page allocation size changed because we no longer have any hash zones for boot zones.