Not all GFX cards are the same, for instance i have a 7970 that acts pretty much similar.
Example my 7970 has a weird sweet-spot, with stock memory frequency 1375 i have to underclock the engine to 843 (from 1000) and then i manage to pull 600Kh/s, otherwise with stock engine (1000) i only manage to pull 500~520k, my Radeon R9 280x (which are rebranded 7970 cards) they all give me a reasonable 710-720Kh/s without a single configuration.
Also my 7970 cannot mine with intensity above 13, with a activity of 64%
or the driver keeps crashing till the machine goes into bsod. But my R9 cards all manage to mine at I:18 just fine, though they pull off 710-720 at I:13 so there's no reason to push them too hard, as too high intensity in my opinion tend to make systems quite unstable.