Although I can not answer your question, why not do some data checks for a few days to get an idea of what you use each day, then go and use Sir/XM for a few days, and check and see how much more it uses each day above normal. This will give you a good idea.

I know I regularly check my data on the my verizon app thing, even though I have unlimited as well. It would not be too hard to check that each day at the start of the day, and write it down. Then after 3-4 days, start using Sir/Xm and continue to check data use on app each morning.

Please report back, because I would like to know too.