SUCCESS!
Even a cheap XFX 7970 can be pushed to performance, and without burning it on 90C temperature!
This is not khash-optimization, but khash/Watt - optimization. So below 700khash/s, I am already very happy.


With VBE7 ... I changed only "State 0 - Boot":

Original:	#0	CC 925	MC 1375	VDDC 1175
MyChange:	#0	CC 750	MC 1050	VDDC 1025

Flashing the BIOS (with atiwinflash) took much more than 1 minute, and the progress bar got stuck at 20% and didn't move for ages - it was terrifying! Finally, it said: Successful; then reboot, no driver update necessary! New cgminer measurements (measured Watt are differences enable/disable, on top of small power for idle card):

The improvement is amazing:

Before:	1.175V	CC  907	MC 1450	72C	fan 75%	267W	619khash/s
1stOptim:	1.025V	CC 1000	MC 1585	72C	fan 59%	222W	691khash/s

So on the same temperature, with less fan speed, I get 12% more khash/s for 17% less electricity.
khash/Watt went up from 2.32 to 3.11 - that's like getting 4 cards coins value out of 3 cards power costs.

YIEHAH!

Now I let it run for a night, to see how stable it is.

This is the way:
0) Reboot, clean system, e.g. no cgminer started.
1) With CPU-Z read out the original ROM, make several copies.
2) With VBE7 change voltage & clockspeeds, save to new filename.
3) With ATIwinflash program the modified ROM, shiver for more than a minute. Reboot.
4) Tweak, fiddle, optimize. A Watt-meter costs 9 euros.

Disclaimer: At your own risk. Inform yourself well.
I spent 2 days of research before I dared to do this. But:

YIPPIEH, I DID IT.
 

P.S.: If you find this useful, you can tip me at: BTC 15UipUK3kGcArJzRyHbVKrwG5TsqYi1b9N


http://cdn.overclock.net/8/89/89350c26_7970-after-flashing.png

I think it's the ADL (http://developer.amd.com/tools/graphics-development/display-library-adl-sdk/) stuff that allows access to temp/fan settings in cgminer. Presumably ADL needs to talk to X for it to work.  Not 100% sure.

thanks you, somehow i manage to get it running by using "-d 0 --gpu-platform 1"

However, i dont see the GPU temp and fan speed in cgminer, it was blank. Also i tried  "--gpu-fan 50 --gpu-engine 1060 --gpu-memclock 1500"  (which was my optimal setting for mining) but there is no change in hash. So i think those command dont work. Afterburner and trix cant too.