I tested underclocking-volting the C1.
The best setting was suggested by another user 2 weeks ago:
675/200 530W 803Mhs 0.66W/Mhs
760/250 790W 1008Mhs 0.783W/Mhs
FYI I have an EVGA 1300W gold.
Could you locate that post please? Your's seems to be the first reference to 0.66W/Mhs on this or the S3+ forums when I search 0.66W/Mhs.
Looking at the MHs i think it is C1. No S3+ can do 800 MHs. And it's to high for S5.
So i'm sticking with guess of C1

I ws looking at the .66 which would be watts per gigahash not megahash? The efficiency would seem very good unless you're off by the difference between megahash and gigahash which then would be quite bad. I looked on the S3+ for the frequency and voltage which might correspond as S3's have the same chips as the C1.
But I couldn't find your reference.
Thanks for warning about MHs. I did make a mistake on not using GHs.
And your correct about same chips. I would guess we will be close to S3 numbers, but cannot say for sure this is one miner I have not underclocked yet.