Which HD 7970s are you using in Linux?

[epixoip]

you should be able to push the clocks on that card to 1100Mhz with no problems. you can even flash the other card's bios onto that card to get the proper voltage tweaks if necessary. but it shouldn't be necessary to do so, we're running all of our reference design 7970s at 1100Mhz on stock voltage.