Netflix works in many countries worldwide now, including SXM. The contracts are different in each zone so you will find some things not available and yet have the opportunity to view things not available to you in your home territory.
I've used Netflix while in Saint Martin, but it didn't give me the same shows as in the US. I believe it uses the France licensed content. One thing you may be able to do to get around this, use a proxy service back in the US or setup your computer at home to act as a proxy server. I've done that before and then had my tablet connect back to a computer at home so it looked to the outside world like it was in the US.
I have used my Firestick outside the USA. It works but says "We noticed that you are outside the USA. Some titles may not be available" We usually don't watch much TV on vacay so i can't tell you what was missing.
Obviously the wifi strength is major in determining how well the shows flow without buffering