Microcode changes don't have to affect performance negatively. Do you have any evidence this one will? If it's a voltage algorithm failure, then I would expect that they could run it as advertised with corrected microcode. Unstable power is a massive issue for electronics like this and I have no problem believing their explanation. Bad power causes all sorts of weird issues.
If it was a microcode bug to begin with, fixing the bug wouldn't need to degrade performance. If it was e.g. a bad sensor, that you can "correct" well enough by postprocessing, it doesn't need to degrade performance. But if it's essentially incorrect binning -- the hardware can't function as they thought it would, use microcode to limit e.g. voltage to the range where it works right -- then that will degrade performance.
