The only thing in your post I really want in Linux is a better bootloader the FreeBSD one is pretty neat, I've seen bootloaders that include kexec to change Linux kernels in early boot. But I seldom see the need for a better pre boot env, and I think this is the general consensus. Also the key to boot speed is to forgo initramfs completely, at least if you look at those sub 500ms boots that are out there.