Bumping the bclk on the 4790 will increase the clock speed of the DMI between the PCH and the CPU as well as the PCI-E root complex on the CPU. Most PCI-E devices don't take too kindly to the bus frequency changing however, the PCH is a different story. You would probably get a different result if you removed or disabled every PCI-E device however, DMI is very similar to how PCI-E works.
IF the PCH is the limiting factor and not the PCI-E devices, increasing the PCH voltage
might get you a little more out of it. However I say this with extreme warning as the PCH is passively cooled and pumping more voltage through it could prove to be unwise. Either way,
@Tatty_One is correct, the PCI-E bus is usually the main reason why you can't get more out of the bclk on this generation of CPUs unless you have a CPU like Skylake with the independent clock gens or, with skt2011(or -3) with bclk straps (an internal multiplier if you will,) but even those are limited to around 5Mhz overclock over the strap (+/- 3Mhz.)
The PCH is a little odd because it usually has more than one voltage and it's uncertain to me which ones drive which components. Either way, I urge caution should you decide to muck around with PCH voltages.