I am currently a 3L at a relatively new law school in the USA. Although I am ranked first in my class, am on law review and moot court, will be published this Spring in a reputable law journal, and have been working at a great law firm doing intellectual property litigation, I feel as though the fact that I attend a relatively unknown law school is holding me back. I am considering an LLM to help open some new doors that are currently shut to students at unknown law schools.

Would an LLM be worth it only if I got into a top tier school? Is it conceivable that I could get into a top tier school?, Berkeley being my stretch school?

I don't know what your career goals are, but if it's to practise law in a US law firm I don't think an LLM from any school will do you any good. Why not try to transfer to a JD program in a better school? Unless you are planning to switch from LLM to JD when you get accepted to a great school, but I don't think that's an easy thing to do. My advise would be to just finish where you are now and try to find some good internships and/or volunteer opportunities to boost your CV. Work experience is at least as valuable as having studied at a big name university.

You just received excellent advice from AnnaLLM. If you have an American JD and stay on for an LLM, people will make certain assumptions. Unless your goals are academia or tax, people just assume you couldn't find a job. This is doubly true in the current economy. The relative prestige of your LLM school may not be enough to counteract this general prejudice. LLMs are mostly for foreign students and academics. Even the lawyers I know who did LLMs to "rebrand" themselves and switch specialty have told me that, in retrospect, it was not entirely necessary. The only exception I can think of right now is if you wanted to enter a specialized field in another area of the country. Even so, it seems like an expensive gamble. In your position, I would try and land a good job. Do well and, in a couple of years, no one will care where you went to school.