I might look at your idea see if it helps at all - I am getting good rendering time with this scene with Sun/Sky - however - right now the render takes 22gig of memory! No word of lie!
22 gig?! Christ! My cards have much less capacity, so my scenes are typically around 4 gig.
That isn't in my card - for some reason that is system memory - even though the whole thing is being rendered by the GPUs - so not sure why the hell it is using so much memory.
Are you sure it's going through the card?
yep 
Then that's weird! Why would all of your RAM be getting used? Do you by any chance have it set on GPU + Cpu?
Nope only the two cards - it is strange it only appears to be happening with this scene also - tried doing it with other scenes and no problems.
You might have 1 or 2 massive textures that are messing it up. I got this product, the comfortable bedroom. The blanket on the bed maxed me out. What I did was I individually rendered everything until I found the culprit. Then in the scene I turned off its opacity. Problem solved.