While the Ksyriums may appear to be nearly a match for the Shimano's on paper, they are an inferior wheelset. I ride pretty hard but look after my equipment well - I had a pair of spokes pop on the rear of my 1 year old Elites, this while the bike was not even being ridden. I have since used both the WH-7850 and WH-7900 wheelsets, and they are superb. Outstanding hubs (much better quality than the Mavic's), lighter, spin up much faster, and definitely stiff enough. The butted and bladed spokes catch the wind less than the wider spokes on the Elite's. The Shimano's are also more comfortable. I liked my Elite's (until they started popping spokes), but the Shimano's are just better. As suggested by someone else, a fairer comparision would be the Elite's vs the RS80, although I would think the RS80 would still take that one too (basically the same wheel as the WH-7900 but with a slightly heavier hub).
I know several big guys that ride their Shimano WH-7850/7900 wheels all the time: training, events, summer + winter. The only problem I know of was when one of them rode into some chunks of metal on the road smashing the rear rim - an incident which obviously would have destroyed any bike rim.