I think the most annoying thing about making your main HD UFS is that your hard drives name after a restart is /. You can change the name, but when you restart it will say / again. I find this annoying.
You can run your Main HD as UFS and run Classic of the HFS, you can even reboot into OS 9, the UFS drive will be as if it weren't in your computer under 9, except that in the Startup Disk control panel, it will allow u to boot off of the UFS.
Hope this info Helps.
P.S. I know nothing about any speed increase or decrease, I just like how UFS has no Meta Data or Creator code junk anymore.