IOH was what Intel called their X58 northbridge, which really only had like a PCi-E controller in it, since then Intel combined the northbridge(IOH) and southbridge(ICH) into the PCH(platform controller hub). However the trick here is that they took the PCI-E controller from the IOH and didn't put it in the PCH, instead they moved it to the CPU for lower latency. The PCI-E controller now sits within the CPU. It lies within the uncore(not core) of the CPU, which is governed by the VCCIO and VCCSA, system agent(sa) should be the one controlling it, however on X79 VCCSA voltage is used for memory controller changes as the IMC(integrated memory controller) is in the uncore/system agent as well. VCCIO would be my guess for the one voltage that might* have any impact on the PCi-E controller. But honestly I say might* because deep down i feel it doesn't impact PCI-E.
If you are trying to raise the BCLK which is now linked to the PCi-E bus(they are the same now at default 100mhz) you can try PCh voltage and vcore.