Because your cost calculations weighted with equipment costs and wattage results in a better earning and shorter ROI on 7950s
I'd like some numbers on that better kh/J claim. Shorter ROI doesn't mean shit if you're planning to mine in the long run.
Enjoy spending an extra $100 per GPU and extra money for electricity costs to get almost no increase in hash rate (if you can even get 700Kh/s, that is).
I'll mine with my 4 GPUs doing 700Kh/s each while you mine with your equally expensive 3 GPUs doing 750Kh/s each.
This
I have a 7970 (msi lightning) , one of the best 7970's you can get and there's bugger all i can do to get it over 650.
My 7950's do 670-680 easy, and that's not even pushing them (I can get over 700 but choose not to due to extra heat/power).