Find out how many shaders you have in each card (my 270's have 1280, not sure about the 280 / 7950) and start low with the xintensity (2 or 3) keeping in mind that xintensity is shaders * x. Use the config for both in your cgminer.conf file (IE: "xintensity" : "2,2"). Get cgminer up and running, and then focus on changing the settings for the 280. Keep bumping up the xintensity by 1 until you see a drop-off in your hash rate, go back to the best intensity, then switch to raw intensity.

For my own trial-and-error, I found 3 was the best intensity (1280 * 4 = 3840) then switched to raw intensity and ended up around 4800 (3.75 xintensity).

Remember to set your gpu threads to 2.

Once you've got the 280 running right, then do the same thing for the 7950. I suspect you'll have to start at 1 or 2. I've got no experience with the 7950 so I'm guessing at this point. If you can't use xintensity with the 7950, you can still use intensity, just hit G, then C (change settings) then pick your GPU, then hit I for intensity. Not sure if you can specify both xintensity and intensity in the conf file (it didn't work for me).

Tuning these cards seems to be a combination of blind luck, late nights and the alignment of the stars.