I use 1.8, 2.0, 2.3 G5 - don't see a significant difference in MorphOS-every-day use. Some differnces may be when rendering (Blender, etc.) and other CPU-intensive programs.
I don't want to have a watercooled G5 - 2.3GHz is the last air-cooled model - it's safer........

Firmware of older G5 (think ~<5.18.f7) allows to use PMacs with only one CPU - which works pritty good under Morphos (needs less energy, produces less heat (also on NorthBridge) and you can safe 1 CPU for spare.
MacOS is also running with only 1 CPU.