i doubt a thank you letter will swing it.
you are either good enough or not.
unless they have asked you to send further information, with which you could combine a thank you letter, or if you have another reason to contact them, i don't think you should send just a thank you.
having said that i doubt it would hurt the application, as long as it was worded well, so i suppose it couldn't hurt.
its just hard to judge how its taken - to some it would seen a polite, to some its crawling...