For what it's worth......
I got my 1991 30 Mk II last December and decided to change the impeller to comply with the maintenance regimen the previous owner followed and documented. I fount NO split ring when I removed it. After playing with it for about 1/2 hour or so, I concluded that there is nowhere for the impeller to go without the clip..........so, like Bob, I also decided to go without.
So far, so good.
Good luck, Merry Christmas to all and a Happy, Healthy and Safe New Year!!!!