Even in case of non-GPU workloads there are high-bandwidth ones, like modern SSDs.
As far as I remember, current DMI bandwidth is 20 Gbit/s - this is 2.5 GB/s.
Some SKUs of Patsburg (C60x) have additional 4 PCIe lanes dedicated for SAS uplink. Where present, this uplink does help with the off-CPU bandwidth a bit.
But where not available, it is quite easy to saturate DMI 2.0 bandwidth with modern SSDs.
According to the public datasheet: http://www.intel.com/content/dam/www/public/us/en/documents/datasheets/c600-series-chipset-datasheet.pdf
X79 does not seem to have the additional PCIe x4 uplink between the PCH and CPU. I am not 100% sure if this is up to date or if anything changed, but if it is indeed correct - then you can definitely saturate X79 chipset when using its PCIe connections for fast storage and such.