As I found out the voltage was locked in the BIOS and you are unable to use Saphire Trixx or MSI afterburner to reduce the voltage.
Luckily i found the following link: https://bitcointalksearch.org/topic/how-to-hack-your-radeon-79x0-gpu-bios-any-version-to-undervolt-it-310434
Which helped me a lot. Big credit to the author swiftshoot.
Basically with a bit of modification i was able to unlock the bios and i am able to change the voltage in trixx.
As i noticed it was hardlock before to 1.175 when under stress (and 0.850 when idling according to gpu-z)
If you decide to follow the instruction take a note that you are doing it at your own risk and i take no responsibility for any damage to a hardware that may occur.
1. download rufus software to create bootable usb drive: http://rufus.akeo.ie/ (use FreeDos option)
2. download atiflash software: http://www.techpowerup.com/downloads/2306/atiflash-4-17/ and copy it to the usb you prepared.
3. boot the pc from the usb. enter the atiflash -i command to check the position of the card that you want to update the bios of. If you have only one card it will be 0 position. if you have more it may differ. it will depend on what pci-e you have the card on.
4 save your current xfx card bios using the command atiflash -s 0 biosold.rom. use different position: eg, -s 1 if different position was returned by previous command atiflash -i. It is mentioned in the link above not to use 256 kb size image but i used it and had no problem with it.
5 boot to windows and make a copy of the bios image that you took before.
6 open the bios in hex editor. I used frhed as recomended by http://portableapps.com/apps/development/frhed_portable
7 conver the value of your current voltage value in mv to hex format: in my case it is 1175 which is 0x497 (use google to convert it or microsoft calculator under windows). now get rid off x and invert the first to digits with two last digits wich gives you 9704. now serch for it in following format:
It should be found twice in the entire file.
Replace it twice with the value that you want to be your minimum voltage ( i used the 850mv, (0.850) as it was the value for idling card and i dont recomend any lower value.) in hex format
8 save the file as biosnew.rom
9 copy it to your bootable usb
10 restart your pc and boot from usb
11 flash your graphic card with your edited bios using the command atiflash -p 0 biosnew.rom -f
12 reboot your system.
Now when you boot to windows you should be able to adjust the voltage in saphire trixx from 0.850 to 1.175. It is possible that with other softwre you would be able to go beyond 1.175 but i did not look for any other software as i don't need it.
I am mining with a difference of almost 100 W having the voltage set to 0.965.
It was around 390W for the system with the card before the bios update and around 290W after.
Good luck..