The only Sonnet upgrade I have is a G4 1GHz, which is a very cheap upgrade for a Sawtooth upwards. That is a straight swap, so no messing with firmware updates, which I suppose adds to the stability. The previous owner of the Cube unit said he always ran it at 1.6 with no problems although it arrived with the dip switches set to 1.5 for some reason. I honestly could not detect any engineering differences between the two. Both slotted in their respective machines with no issues.
The only issue I have had with an upgrade is with a Powerlogix Blue Chip G3 daughtercard for a Wallstreet. Cannot get that one to work in any I have tried. When I plug the power unit in the Wallstreet with it in the LEDs stay lit and the the fans race. I can force a shutdown from that but it will not boot. Prolly dead.
The only issue I have had with an upgrade is with a Powerlogix Blue Chip G3 daughtercard for a Wallstreet. Cannot get that one to work in any I have tried. When I plug the power unit in the Wallstreet with it in the LEDs stay lit and the the fans race. I can force a shutdown from that but it will not boot. Prolly dead.