myRect.x = 10; // Currently this line will not cause the variable to sync. Please support syncing data when individual variables or a structure change.
myRect = new Rect(10, myRect.y, myRect.width, myRect.height); // this line currently causes the structure to sync. But it is inefficient to be constantly calling new and synchronizing all member variables of the struct needlessly.
P.S. As a low cost improvement Unity can do immediately, you should also explicitly update SyncVar's documentation to show examples of proper and impropper usage of SyncVar for syncing structs.