Okay, let me explain it again in SMALL WORDS.
VDDCI is memory controller voltage. Not memory voltage.
Memory voltage (MVDDC/MVDDQ) is from the 2 non-software-controllable VT243s (VT237s on 5970s).
Non-software-controllable. As in "needs hardmod to change".
now, on to VDDCI, which *is* software controllable on reference 5870s and 5970s.
It's already possible to play with it in linux with a trivial modification to radeonvolt as it's a vt1165 like VDDC, just at i2c addr 0x71 instead of 0x70.
So... how much power can you save lowering it? on a 5970 mining @ 300 memclock, reducing it from 1.05 to 0.90 drops total power/card by ~ 1W...
Yeah.