After several tests on my single gpu M18x R2 and @Infinaris SLI setup, we have come to a workaround for the throttling problem occuring with the 980m. I am not sure if this can help 780m SLI or 880m SLI systems, but they are welcome to try.
Do the following and post results.
An unlocked BIOS is needed and one can be found from here or...
Alienware M18x R2 with 980m - Possible throttling fix
Do the following and post results.
An unlocked BIOS is needed and one can be found from here or...
Alienware M18x R2 with 980m - Possible throttling fix