Where did you read that using the BIOS switch voids the warranty? The BIOS switch sets the clock and voltage to HD 6970 level and the card is design for that. It does not make any sense to put the switch there if it voids the warranty.
Yes I believe some crazy OC which would need much higher voltages than the switch provides could void the warranty.
What voltage settings did you use for the 970/1300 OC? Why did you use 100% fan speed even for reference settings clocks.
Anyway my HD 6990 is in the mail so I should find out soon by myself.
EDIT
Just finished reading the
HardwareCanuck review about the HD 6990 OC, you need to use the switch for higher OC also didn't know about the CCC Power setting slide which is new, probably for the HD 6990 only.