Generally speaking there is an increase in rigidity, though not by much. Was it worth 600? I don't know for sure. But i'm satisfied with it.
I wasn't ready to do more than these bolt on though as the car is mainly used for traveling and I expect it to retain the spirit of the M550i - this dual purpose car, comfortable saloon for traveling, guided missile when you want it to be
I learned my lesson on the F30, where everyone was raving about coilovers which in real life were making the car both hard to live with and less stable (= lower chassis performance) due to excessive stiffness.
I like your approach though, I believe changing the G30 suspension parts with F90 can bring noticeable handling improvements, without compromising drivability and stability, I don't think I would go for KW coilovers though.
The rectangle piece is wielded to the cross strut, though that doesn't show on realoem diagram.