I'm currently running a 750 watt PSU (corsair tx750) and I'm contemplating SLI'ing gtx 770's in my system. As of right now I have a single 660 ti with a system load of approximately 275 watts. I know the 770's have a tdp of 230 watts (ish) and with two of them in my system I'll probably peak at around 500-525 watts system load.
At 525 watts my PSU will be running at 70%.
I've read that the PSU efficiency spot is between 30-70% of its load, but I'm not sure how accurate that is.
From my understanding a PSU is most efficient somewhere between 50 and 90% load (in compliance with 80+ Certification) so I would assume its roughly 70% where its most efficient.
You could always check reviews on the PSU, JohhnyGuru is the big name in PSU reviews and would feature an efficiency curve in his review.
Though what may be more important is how efficient it is at loads at or under 20%. Thats where your computer will be idling at.