There is no "sleep mode" for the psu. There are no modes at all. It gets a call for power and supplies it. If the pc wants a little power it gets a little power. If the pc wants a lot of power, it gets a lot of power. What makes it non haswell compatible is when the power gets too low, voltage goes out of spec and protections trip. It just can't regulate voltage correctly when such little power is going through it.
The power off the wall is an irrelevant concern. It's always going to be the same voltage and amps changes from load. You'd fear from any electrical device blowing up from what it sounds like but that's not how electrical devices work. The pc has no load, then it can't be overwhelmed. Only if the pc wants more power than the psu can handle will it be possibly overwhelmed and trips high current protection or temp, volt, etc. But this is on the opposite side, there's very little, almost no power. It's really getting underwhelmed and tripping on low.