My understanding is that the 9150 models are compatible with G3 upgrades, however, they will not work with the PDS pass-through option of the G3 cards. And the only reason they won't work with the PDS pass-through is that the PDS pass-through ribbon cable only bends in one direction, and the PDS slot in the 9150 is on the wrong side of the NuBus slots, i.e. opposite the orientation in the 7100 and 8100.
The Power Computing 8100 clones have the same issue. They work great with G3 upgrades, but if you try to install a PDS pass-through option so that you can use the HPV card or AV card, the option clunks into the bottom (left side in desktop model) of the case.