The Si stands for silicone which they modified the escapement part,
The uni-directional winding of the 7750 still remains the same, which means the wobble will still around but would be lesser compared to other 7750 watches as it comes with a solid back case and a soft iron cage (for dc56 yes but not sure for dc56 si*which si is already anti magnetic) to shield movement from magnetic field.
Casing wise very good review from and feedback from majority of the owner. Hardness of the case is higher than stainless steel, theoretically any material that is higher in hardness could scratch the one lower. The ice-hardened case is 800 vickers which is alsmot 4 times of the 316L stainless steel hardness.