Thanks for the info.
My locked Gigabyte 7950 WF3s do not drop nearly as much in power consumption when underclocked. See the lock goes both ways. I cannot OV nor UV. 3d mode means 1.15V for me, be it 800Mhz or 1100Mhz.
This is with the FX1 / FX2 BIOS? I managed to flash F43 on mine despite people saying that the boards with FX1/FX2 don't work with F43 (mind you haven't tried HDMI which was one of the things which some Gigabyte BIOS revisions had problems with). A bit risky of course but the card does have a BIOS switch...
Anyway, while the voltages are still limited, F43 defaults to 1.09V which is a lot better than the 1.25V the boost BIOS had. Only when underclocking to 700Mhz does it allow 0.95V. Not ideal but better than constant 1.25V.
BTW: while I can't adjust memory voltage and I didn't look too much into this I did measure the power difference of memory clocks:
1100 MHz 1500 MHz 1090 mV 245.5 W
1100 MHz 310 MHz 1090 mV 210.5 W
Those figures are my estimates of card usage. That is full load less (estimated) mobo/CPU etc. with an 85% FXF (Seasonic based) PSU.
So, GDDR5 uses a fair bit of power - too much to be useful for main memory. So laptops with PS4 type Jaguar CPU/GPU is probably not a good idea.