Measured the output voltages from the +12, 5 and -12 spades on the bus boards. All within .1v, and no sign of any spike at power on/off. So looking good I guess.
Weirdly, I noticed that in my other case with factory-installed PSU3, the LEDs will flash at power off/on when it hasn't been turned on in a while, but if I turn on/off again a few times they stop flashing. So who knows As long as my case is safe. Thanks again for help!