Hi Rolf,
That pic was taken early 2005 as I recall, during a club race. We are beam reaching (just too shy for the kite) to our West Mark, in about 12-15mph of wind. It was just after this that I replaced the carbon boom with a stiffer ali one. We've got almost 180Kgs out on the wire!
That huge stern wave is more than likely caused by a bit of weed around the rudder - a permanent problem when our courses take us down into that corner of our sailing area!
In terms of displacement/planing arguments I'd hesitate to suggest that that pic proves anything!