Interesting.
I have been mining with my gtx 670 for a while getting about 150 - 180 kh/s , and only recently with the new version on cudaminer i have gotten up to over 250.
I have thought about getting a R9 280X to get better hashing power to increase the gainz
But I'm wondering if this problem that OP has often occurs? Does it happen often that you can get a card that has lower performance than normal?
This weekend I plan on mounting all the guts onto a platform and running all 4 of them, see how it performs when it's dedicated.
I'd really like to get an update to this^, as it would kinda answer my question