The fact is that even voltmodding, the overclocks you can get for 24/7 won't give you noticeable performance increases, as in the framerates don't scale linearly increasing the frequency.
In fact i think that voltmodding makes sense only if you are trying to beat some MHz/benchmark score, or trying to uncripple a card such as a HD4850, increasing its low memory performance.
P.s.:From those pictures the power section looks totally different compared to the HD3870, if it's not a nonstandard
PCB card, the HR09 Type 3 would never fit, it would be too short i think.