After 20 minutes of testing:
first 7950 from 1.97 Mhs to 2.77 Mhs.
second 7950 from 1.88 Mhs to 2.64 Mhs.
one 280x from 2.22 Mhs to 2.65 Mhs.
So this rig (2x 7950 + 1x 280x) from 6.0 Mhs to 8.0 Mhs.
No settings changed. No temperature changes.
Not bad. Strange that 7950 gets higher Mhs than 280x ?! Maybe can improve a bit more...