Having returned through ATL on Delta many times, you should be fine unless there is a significant delay leaving SXM. I would not want much less time and it also depends on the day of week you travel, C & I staffing and the number of other international arrivals that came in before you. Luggage is usually not the issue; it is how crowded C & I will be (it was crowded in March when I returned and took around 1 hour)) and then getting back through security to get to your departure date.