I did it once but a little different. I de-soldered a ATX connector off a dead mobo, cut the ATX connector with as much wire as available (about 9") off a dead ATX power supply and soldered the cut wires to the connector from the dead mobo to make an extension. That way I didn't have to butcher the wires of a perfectly good power supply.
I never seen any negative effect from that. All the rail voltages seemed to be were they should have been. :shrug: