I've just committed the first patch to the git tree that should enable 79xx cards to work properly under cgminer (delete any dodgy .bins created before this attempt). I have no idea if it performs even remotely well, but in theory at least, it won't behave stupidly as the current version does. If someone does run it, could you please dump the output of -D -T the first time you run it? Also it may be worth starting it with -v 1, and/or experiment with various combinations of -v and -w (default being -v 2 -w 128 if you set nothing).
I tried to test your 79xx patch on the git tree following the instructions in the README but after several hours of trying I was unable to get it to actually build on my windows machine.
But it might help you to know that I've found that using the poclbm kernel, setting vector size to 1 and hard-coding the poclbm kernel's BFI_INT to false gives me ~600MH/s (which is much better than the ~540MH/s v2.1.2 kernel with the same settings).
Further gains can be had by changing the definition of ch() and Ma() in the poclbm kernel to use bitselect() instead of boolean operations. Changing this gets cgminer's Radeon HD 7970 performance up to within 3% of that of Diablominer's (diablo gives me 670MH/s while cgminer gives 650MH/s @ 1125MHz).