I haven't tried 16.04R yet, but I have a few suggestions for 12.04R that may also be relevant to the former...
- Update Yaboot to 
1.3.17 over 1.3.16.
- Add 
windfarm_core to 
/etc/modules. On USB, my G5's fans were blazing without it.
- I haven't tested, but add 
pmu_battery to 
/etc/modules for laptop battery recognition and monitoring.
- Add 
vm.swappiness=10 and 
vm.vfs_cache_pressure=50 to 
/etc/sysctl.conf to make swap management more efficient, especially on machines with less than 1 GB RAM.
- Add 
Acquire::Languages "none"; to 
/etc/apt/apt.conf.d/00aptitude to make APT operations faster.
- Preinstall 
preload to increase speed.
- On 16.04, preinstall 
Surf and 
SurfTube so that people have a fast and easy YouTube player, plus a lightweight WebKit browser OOB.
- Remove apps like Simple Scan, and the Penguin games to lessen image size. I'm pretty sure nobody wants to play the Penguin games.
- MX Linux uses 
sysvinit instead of systemd for its default init, which also improves performance. Maybe you could do something similar, just by installing and enabling it? It may well outright fix the graphical and sleep issues present on the G3 / Radeon machines, so I would at 
least look into it.
Otherwise, it's an all-around great experience to have (mostly) everything working OOB w/ our great Web browsers already installed. We've got some fantastical stuff here, so thanks again to you (and the rest of us, too). 
