slocaus, i'm pretty confident you are wrong.
I have had rockbox installed on my H320 and after using it for around a month on a brand new 2200mah replacement battery I noticed (when flashing back to the original firmware) my battery life had been reduced by 4 to 5 hours.
I actually then went out and bought a 2200mah newertech battery.
Ran the original firmware, volume 20, a folder of 128kbps mp3s repeating in order continuously without turning on the backlight, and got 27 hours of playback time. After using rockbox for a few weeks and only about 10 charge cycles later the same test would only run for 23 hours with the same firmware back on there, as when I ran the initial test.
Since I sold my H320 and bought an X5L (which isn't as easy or cheap to replace the battery on) I am definitely not installing rockbox until they fix the battery issue. I don't care what anyone says, claiming rockbox doesn't wear out the battery quicker.