I've update to the Beta drivers 340.88 and the hash increased slightly. Now getting 7.1 Mh or so but still not the 7.8/7.9 Mh you are getting. I also tried with minep.it pool but as I was expecting it didn't change anything.
Any more ideas?
I have not followed the whole conversation, but I am wondering if one of you is running on risers and the other not?