Not quite true. Depends on "just fine" and what it means.
Here's the SLI performance of a GTX680/670FTW based on 8x vs 16x (battlefield 3 was used for these):
http://i.imgur.com/7ZV3K.png
Note: 8x/8x PCIe 3.0 = 16x/16x PCIe 2.0/2.1 (literally, the bandwidth of 3 is double that of 2/2.1)
Single monitor gains aren't that huge, but they're still there. Anything beyond one monitor is a clear gain. Of course, years time to SLI means it'll probably be new mobo/cpu time anyway. If you plan on single monitor, single video card for a good amount of time, the x79 has nearly nothing for you unless you chug large datasets or something.
Bottom line: Performance/cost ratio - z77. Raw performance - x79.