Considering I was just in SXM this weekend...
Customs were quick, under 10 minutes once I got off the aircraft.
Baggage came out quickly from what I saw (I didn't check bags for a day trip).
You're spit out of the secure area, at one end of the terminal, and WinAir check in is at the other end. 5 minute walk at most. There is a separate customs area for those connecting, but since you don't have onward BP's, I don't think you can use it.
If the flights are on time, I think you'll be okay.