Thanks for advice. Semi sorted out the problem, and submitted a bug report. For posterity, posting results here: 8x PCI-e bandwidth problem on resume was because I hadn't selected "load optimized defaults" and pcie voltage was set low. However there are almost definitely two bugs in the BIOS I have found:
1) Since implementing the "Improve USB 3.0 Compatibility" fix in F12, resuming from sleep with the USB 3.0 controller disabled results in the problem described above. Black screen, CPU fan off, need to unplug system. It has nothing to do with the turbo setting. I thought it did, since I was disabling both at the same time. On BIOS F10, everything works correctly (solved the 8x pcie on resume problem as described above).
EDIT: 8x PCIe bug STILL HAPPENS on F13,F14, even with optimised defaults selected and voltages set correctly. Works fine on F10, which is why I thought it wasn't an issue. This is still probably a bug on the new BIOSes, probably dealing with the USB3.0 controller, since that knocks the pcie slot to 8x mode when operating at full speed. To be clear, with turbo disabled for USB3.0, pcie slot should operate at 16x, which it does, until you suspend and resume on the F13,F14 BIOSes. Then it's down to 8x. Can't disable it entirely, as then you can't resume at all! Again, F10 is fine as far as I can tell.
2) The 1.15V vtt option is gone from F13,F14 (and possibly F11,F12) BIOSes. F14 is the newest. It's there on F10, and I use it. On newer ones, it goes from 1.10V to 1.17V and then up in .2V increments from there. Submitted that as a bug report, too.
Can anyone with a P55A-UD4P confirm they've seen these issues?
