I really don't know, I've never read any (semi-)official word on this other than that we shouldn't assume that any 3rd party CPU upgrade would be supported.
> As far as I've heard Powermac support is also tested on a > Powermac 3,1 with a cpu upgrade card.
Nice if it's true. Do you know which CPU card that is specifically? An MPC7448 card would be too much to hope for I guess ;-)