Honestly, it's not that big of a deal. Theoretically, PCIE 3.0 x8 has the exact same bandwidth of PCIE 2.0 x16. Extensive testing by TPU's W1zzard proved this so with a GTX680 and HD7970 on an IVB platform. His test reults show that even running both cards ran at full PCIE 3.0 x16, there is a max of 1% performance increase over 3.0 x8.
Granted he tested only a single GPU setup, I wouldn't imagine you'd be able to saturate all of that bandwidth even with 3 GTX680s.
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