core voltage cannot be tweaked...
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.
you can try edit VBA BIOS. try google RBE. but it can damage your card.
i recomend use sme windows tool to overclock your card to find out maximum. then edit BIOS and set this values into.
but this is DANGEROUS and it can damage your card. you are doing it at your own risk.