I didn't used atiwinflash but i would say it "might" work. The important thing to notice is that you can't write a bios correctly on machine where drivers are installed. I know it because as you I failled to flash the bios this way. The result is a bricked GPU and there is no other way but flashing the original bios back, if possible.

So the good reciepe for flashing is : get a compatible bios from techpowerup, flash it using a win98 usb stick in command line. The atiflash.exe I used was v3.99 as 4.17 was not working.

Can you give me the reference of your bios like 015.022.000.001.000000