I have used the fw800 cpu (dual 1.42ghz) chips in the fw400
and it works alright - very quick for macos9 - only that it does run hotter
and the os9 doesn't have good manual software fan controllers -
the rate of the fans seem to be higher and noisier in os9 than in osx.
This is probably why the mdd 1.42ghz processors were on a model
that were macosx only the fan controllers were able to be controlled
better within the system - I don't know the reason.

So if I can use the cpu for fw800 on a motherboard for a fw400 mdd and macos9
works then the ROM for os9 must be on the fw400 motherboard and
not on the motherboard for the mdd fw800 g4's - my guess.

So the only downgrade would be a motherboard of the fw400 version
where the macos9 ROM chip resides. Must not be on the Fw800 model
motherboards.