the 6700's are fine for stiffness. Not as light as a the rotors but down there you won't notice that you dropped a few grams except when you actually set the bike on a scale.
This Fairwheel test is not your 6700 crank but you can take some of the comments and results of that and compare to the rotor.....other than a few more grams that an Ultegra will carry over DuraAce, Shimano shares design features/methodology well amongst neighboring models and generations of equipment....meaning I'd suspect that the 6700 stiffness will be excellent and same rave over shifting with Shimano rings. http://fairwheelbikes.com/forum/viewtop ... 3e9#p99937
Unless you're now thinking of losing grams from the crank (original post was about getting a stiffer crank) I'd save the dough for something else that will provide a real benefit.....something such as (assuming you don't have one of such already) light and aero wheels, lower rolling resistant tires, heart rate monitor, power meter, etc