If I remember correctly I saved about
150 Watts Total going from
3*7950 @ 1.250v to
3*7950 @ 1.087vUndervolting does not affect you're hashing speed directly.
Though with a lower core voltage you will not be able to clock you're GPU clock as high as with a higher core voltage.
In my case it did not affect my hashing speed at all.
First I found my highest Memory Clock speed and its correlating GPU clock.
Only then as a final step I lowered the voltage.
I wrote a guide on how to overclock/undervolt you're 7950 and 7850 in Windows 7/8.
https://bitcointalk.org/index.php?topic=175646.0