Been running all night at 669KH/s, so close!
I'm using 4 as XIntensity (not the normal -I) and two threads with -g
Since the 7950
1 got 1792 shaders, X:4 should equal 7168 threads and X:5 should equal 8960 threads.
Intensity 13 equals 8192 threads, so neither X:4 and X:5 hits it exactly.
You can use --rawintensity for even finer control, but it's hard to guess what would work though.
I'd start with 7168 and work my way up, maybe increase by 64 every time.
1 7950 specs:
http://www.techpowerup.com/gpudb/307/radeon-hd-7950.html