I never directly compared battery life w/rockbox vs. w/o. All I know is that I never have managed to deplete the battery of my H120, especially since I upgraded the battery to a Cameron Sino 2200mah.
Rockbox does crush the original firmware. I wouldn't hesitate to upgrade to it. Make sure you follow the procedure for flashing it to the iRiver's ROM, and booting from there. It makes the player boot up in less than a second. Even if the SQ is the same, Rockbox is so much easier and more convenient to use, it isn't even funny.
On to the main topic at hand, if you don't need a DAC in your amp, I would definitely wait for the Pico Slim. The volume control on the prototype I tried at CanJam is the most precise and flexible for low-level listening (tested with the super-efficient Shure SE530's) that I have ever tried. Including the prototype Shadow.