I'm not sure that the expander plug will help since you are intending to run the stem above the top of the steerer tube and the plug will not fully support the tube from the inside. Without a spacer, the top cap will need some room below the top of the stem to fit. Not that it matters, but I have my S2 stem installed without a spacer (using the star nut) and it is OK after some 13,000 km. Another possibility is to get a stem that has only one clamp bolt, such as the 3T ARX II Pro. I'm using one on my S3 and clamping is not a problem with the single bolt. I guess the other possibility is to flip the stem to get the bars higher without moving the stem higher on the fork.