i've succesfully done the vmod to my non-reference 4870 512mb, an OC edition by club3D.
as you can see here
unfortunately the vdimm & vddq is controlled by the same (single) vrm so overvolting memory makes absolutely no difference on clocks because the chip can't cope with the power requirements.
feeding more voltage to mem causes only instability.
the core has got 4 vrms instead of 3 of the reference, this makes the chip clocking quite nicely 920@1,37v, i'm on custom watercooling (the chip idles at 32°C and loads at 36°C
the card is not so much memory limited so i prefer having nice clocks on core instead of mem.
i forgot to say that the soldering point on this card is the same of the reference one but the place is covered by paint so you have to scratch a little to do the job.
here you can see the pwms of this card.
hope this might help someone out there.