I am in Asia and have been up all night;) Thoughts are a bit scattered. The bios switch and button have nothing to do with slave vs master here. There must be a way in ati flash to specify which you are flashing. The devil clocks are not drastically higher, so hopefully no huge risk. But you had better watch your temps like a hawk. If they go high, then I would revert back to the stock bios. These cards are hot to begin with.
I would suggest posting to Ace123 for a step by step guide. If he has done it and it works, then better to get instructions from him. Every card is different when OCing. He could have been lucky to get a non devil version that happen to have perfect components.
I have 6 7970s and two OC very well with no effort, the others are a pain to keep stable.