Would enabling the setuid bit on a shell script make a difference at all when that script is run on boot? Who would the effective user be?
No, setting the bit would have no effect during boot. During the boot proper, all proccesses run as root. As daemons are spawned, some are run as the appropriate daemon user, but unless your script is called by one of them instead of the init scripts you don't need the suid bit.
Setting the setuid bit on a shell script has no real effect ever (except during specific permissions checks by other programs), as shells are generally not configured to run scripts setuid.
The s permission has no meaning for shell scripts you must use the "setuid" system call in your program to do that but The operating system does not support the setuid or setgid subroutines within a shell script.