I seriously doubt the CR350 is as stiff as the ksyriums. The rim is much lighter and has a weight limit on it. If it was so stiff then why so many spokes? The stiffer the rim, the fewer spokes need to support it. The CR350 might be stiffer laterally due to high spoke count, but vertically? Not to sure about that. But I've never ridden them so I might be completly wrong. Anyone own these care to comment?
If the rim depth is the same (CR-350 is 24mm deep) and the wall thickness as well, then it will be as stiff vertically. Laterally it will be stiffer with a higher spoke count as you suggest.
How many cracked Ksyriums have been heard of? I've heard of (and seen) quite a few of them. the 24mm CR-350 would avoid this by having more spokes sharing the load on the spoke holes among more spokes that are also more flexable (better spring rate - the reason why double butted spokes are better than straight guage). The CR-350 built up correctly may just be a better wheel than the Ksyrium, although it won't look as cool, although what I've heard about Spada using them with black CX-Ray spokes might tip that ballance.....