I get the feeling its poor electrical design combined with lean euro tune. When you move the pedal, I think the
ECU is getting the throttle position from the actual throttle instead of the pedal. Now when you WOT from cruising speed, the throttle opens faster than the TPS can read and can't richen up the lean mixture quick enough, and that where you get the ping. Perhaps better grounding could help this?
I also think the injectors or the fuel pump are only just sufficient for the job, or the possible lack of a return line (to purge air out) or bleedback valve. This also explains the problem with starting on hills where all the fuel has drained out of the lines and left the pump high and dry.
I'd like to see a aftermarket higher rating fuel pump installed into a Focus.