Well the G6 may likely have had the power saving features that made a PowerBook G6 possible.
The Power5 included CPU-based dynamic thermal/power management.
The G5/Power4 needed a service processor to control this feature, the Power5 ramped up the thermal diodes by a whole lot and did this by itself and a ways ahead of PowerTune.
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IBM was quite willing to do it for several $100 million bills, and more of those if IBM ran into production problems.
Basically they send in the IBM production techs to fix the fab problems quick with this result...
IBM Looted My Project Budget