What I meant to say is that I agree with blacmercedes on the rock solid thing.
Anyway, my only complaint with the C at high speed is the wind noise. I guess I'm spoiled with the double pane windows of my S420.
Of course the 140's are in a complete different league when it comes to high speed (can't hear or feel a thing), but the C is a much smaller car.
However, I'm not having any of the problems that you guys are describing (shifting, movement and stuff).
I think that the biggest factor for high-speed stability are the tires. Honestly, I wouldn't feel safe going over 100mph with my stock rims and H tires.
I use 17"x7.5 F and 17x8.5 R with Potenza tires. The ride is a bit harsher but the handling, braking, and high-speed stability are phenomenal.