My main PowerMac G5 is the 2.7Ghz dual CPU model. It maintains ~86C very well with minimal fan noise. It is actually pretty quiet. It is pretty clear to me that the default thermal setpoint for the 2.7Ghz model is 86C. Considering the age of this computer, I'd like to lower that a bit, even if it means more fan noise. I could probably mitigate that anyhow by changing the fans with a quieter type.

Is there a way to change that setpoint? A quick Google search was not my friend. TIA.

You could power the fan from an alternative source, such as a molex. That would mean it would run 100%. If you wanted to slow it down somewhat, there are fan controllers, both manual and thermostat controlled.

But as to adjusting it in the BIOS, it's probably hard-coded. Apple don't like people playing around with things.