Hm, but it's using the same phone number so I think that the Sim will register with the Sim in your phone. Setup will all have to be done from your phone.
I don't think that you will need a new plan, I think that it will use your current plan.
Or did I get the wrong end of the stick?
Honestly, I really don't see the benefit of going through the additional burden of getting a new cellphone plan, SIM, paying more for ... what? One advantage is definitely that in emergency situations (i.e. on a run without the phone, hike etc) you can now call from the watch. But I just don't see any other benefit.