hi, I've pm'd Darkhmz, but thought I'd post my findings and questions here too. I would appreciate any comments please.
I recently purchased two XFX 7950 cards (FX-795A-TDCK 925MHz) and found they were getting very hot, due to the boost function. They are factory over clocked to 925MHz, but easily reach 1200MHz using Trixx. One had quite a low ASIC quality around 69% and boosted to 1.25v, the other was a bit higher at 76% and boosted to 1.169v. I was disappointed at how how hot they got and I'd previously used an Nvidia BIOS tweaker on my GTX680, so was keen to do the same on the 7950. I found this BIOS editor and gave it a go.
Unfortunately my cards don't have the BIOS switch, so I knew this was a risk. Both cards have the same SKU and ordered at the same time, they appear identical except for the ASIC rating, I'll need to check the PCB number later as I'm not at home currently. I was using the highest ASIC rating in the top slot in my PC as it boosted to the lower voltage (1.169v) and was slightly cooler.
I first used Trixx to overclock the cards just to benchmark, but reset them prior to playing with the BIOS. I saved the original BIOSs first. I did a test flash of the original BIOS to the 2nd card (lower PCIe slot) and all seemed well (although I didn't try it by itself at that point, it was in crossfire with the primary card). I didn't use the same BIOS for each card, I used their own BIOSs both original and edited individual BIOSs.
I used ATIWINFLASH with the -f -p options.
I used the BIOS editor to set the voltage to 1.069v and freq to 800MHz to be safe. Then used Trixx again to see how high I could get the freq at 1.069v, which was 925MHz. I then reflashed the 2nd card set to 1.069v and 925MHz.
I used the same method for the 1st (top) card. But for some reason it ignored the frequency and fan settings, and only used the voltage setting, so I set this to 1.069v too and all was good at the default 925MHz.
At this point Trixx was not loaded or used. CCC had no o/c set.
It was when I swapped the cards over to see if I could set the top card's freq and fan settings with it in the lower slot (so the 2nd card was now in the primary upper slot) that the system would not boot even into BIOS, even with on-board graphics set. I tried it with the 2nd card by itself in any slot and it would not POST.
I flashed the card with its original BIOS and it still would not POST. It would only POST when the first card was in the top PCIe slot as the primary graphics card and the second card was back in the lower slot. Even though the first actually now had a non-stock BIOS!
So it's quite strange that this second card won't POST even with it's own original BIOS, yet works fine as the secondary card in crossfire. It's also strange that the first card completely ignored the frequency and fan settings (hence I had to use the default 925MHz), yet I could set the voltage fine.
As I say, the system is working fine in crossfire, and I'm extremely pleased with knocking 20% off the voltages (and power) at 925MHz and the significantly lower temperatures. But in the furture I might like to sell the cards so ideally would like to fix the issue with the 2nd card.
Many thanks for any help you can offer.
Here's my stock BIOS
https://dl.dropboxusercontent.com/u/47847501/1st_slot.rom