Isn't OS9 supposed to supported by OS X? Os9 even has an unused Carbon extension and Apple boasts OS9 on forward compatability. Not sure but I'm hoping so.
So far I'm happy with 9. I've got 8.1 on my other internal Barracuda in my 7500/200 604e.
I think by the time OS X client comes out, I'll probably get a new Mac and keep my 7500 around as an additional workstation and drop in a cheap high end G3 upg.