Yep, agreed, it performs well - given that it's flying from the DVD and isn't properly optimised. I monitored CPU use across 720P playback and web browsing (no video playback on Youtube - probable GPU issue) and it's pretty close to OSX - although out of the box, there's no audio - that adds plenty of cycles once it's set up.
However, Lubuntu/LXDE is very basic compared with the luxury of OSX - so performance might dip once LXDE has a dock and enhancements etc.
But yes, if you can survive without the Mac core apps and visual/audio creative apps then Lubuntu sits well on a Dual G5.
Funny, I've never had any problems with audio on Linux. I might have to double check, though.
There are plenty of alternative apps. I've heard they're quite good.
Question, if you have the Lubuntu 16.04 DVD, when you're booting it up, after yaboot, but before the desktop, does it show the Lubuntu logo with the white dots against the dark blue background? Mine doesn't. Just a dark blue screen.