core voltage cannot be tweaked...
-INFO-
via aticonfig you can edit the gpu core and memory clocks.
There should be info and examples printed out by aticonfig --help, but that is pretty verbose and hard to find what you're looking for.
The commands are as follow:
Aticonfig --odgc to get current clocks
--odsc to set clocks
--ode to basically sign a waiver to begin overclocking
--odcc for commit clocks.
Basically saves new overclocked values to the registry for persistence
--odrd for restore default, to go back to stock values.