1080p.
Well it's no wonder you're getting less than idle SLI scaling. That resolution can be handled by one 680 (most games) so the extra 680 is going to result in underutilization perhaps even negative scaling in some games.
For new games without a mature SLI profile or if you don't know how to set it up using nVidia inspector then you'd perhaps get better performance using just one card.

, I realise nothing solid yet but keep running it over in my mind, what the max I would really pay for something like this 

I keep hunting but the same ol rumours we already know keep popping up. Hopefully something solid soon 