I did some measurements on both PC (connected to a plastic case wall plug 5V2A rated voltage power supply, actual output voltage is 5.36V), and PC2's (connected to the SMPS mentioned above, actual voltage is 5.40V) from GPIO pins and USB cable ends under load (running stress command):
I get exact same 5.36V on bot GPIO pins and USB cable ends on PC
I get regulated 5.23V on GPIO pins and 5.23V on USB cable ends on PC2 (5.40V->5.23V)
As I use these SBC's for headless server use with no USB device attached to them, haven't encountered to any problem. But you may be right that 5.36V is dangerous for USB devices though.