My board (DZ68BC) also supports ME 7 and 8. I have not been able to find Intel docs that state how the BIOS will decide to select ME 7 or 8. It may be the board revision or the CPU architecture: Sandy bridge vs Ivy Bridge. My assumption is that the CPU architecture controls which ME revision is activated. I have a sandy bridge, thus ME 7 is activated.
The assumption sounds plausible.
But I would like a confirmation before spending the money ;-)