1) Will thank you notes improve your chances?
2) If you are a senior, will your grades matter after you start interviewing?
3) What are the things you can do to get yourself off the waitlist?
4) Questions #1 and #2, now assuming that you are on a waitlist
As mentioned, these are actually questions that get asked multiple times every cycle. The fact that you think nobody will ask them suggests you need to peruse SDN a bit more thoroughly before you type.
1. No it doesn't help but that's not really the point of thank you notes. Thank you notes are an issue of professional manners. They are not meant to help you, they are simply etiquette -- the way you behave when you are on a professional road. Sort of like holding the door open for someone -- you don't expect to get anything from them, you do it because that's the kind of person you are. It won't help you, nor should it. The reader will spend two seconds looking at it, say "how nice" and dispose of it. But that doesn't make it any less the etiquette. Being a professional is not about only doing things you are required to do, or will help you. It's about conducting yourself in a certain way.
2. You can send update letters with good grades, which may give adcoms a reason to open your file again, maybe reconsider. It also should be noted that about half of all applicants don't get into med school, so if you end up a reapplicant, all the grades will come into play.
3. LOIs and update letters.
4. Again thank you notes are not about helping yourself. See #1 above. It's sad that folks don't get this, notwithstanding that it is answered year after year. As for grades, see #2.