I am finishing in the bottom half of my class at a tier 3, but was accepted and will be attending a top 20 for an LLM. I know how a sub-par GPA from a Tier 3 is viewed by prospective employers, but does anyone have any insight as to how much getting an LLM from a Top 20 will do to improve my "status" and/or how I look to employers?

Here are a couple of articles that you might want to check out:

http://abovethelaw.com/2012/01/the-value-of-the-ll-m-degree-still-low/

http://www.law.com/jsp/nlj/PubArticleNLJ.jsp?id=1202537948154&Big_law_firms_dont_care_about_your_LLM_recruiter_warns&slreturn=1

Tax LLM seems to be the way to go to if you're looking to improve your status.

My opinion is that it were an LLM at a tier 3 it would probably not do anything. However, going from a Tier 3 to a Top 20 should significantly improve your standing, as it would show that you are capable of competing with the best. In addition, a Top 20 school can offer so many more opportunities (career center, networking, etc) that Tier 3's do not. Finally, bear in mind that schools in the top 20 typically have employment rates over 95%.
But that's just my opinion.

I think 95% employment rate might be misleading. A lot of llm candidates are foreign grads with years of experience, and a good percentage of those that have jd probably also have substantial work experience or secured employment prior to entering the program. Also, there are those who are already working and their employers are paying for the llm. I wonder how many among the 95% went straight into the llm program after getting a jd and without any job prospects.

I have read those articles, but I hope that they are just the fringe opinion, rather than mainstream.
In my case, I have no work experience (beyond summer internships), having gone driectly from undergrad to law to LLM.
I know how I look now to employers with a weak GPA from a tier 3. I'm trying to figure out just how I will look to employers after getting an LLM from a top law school.

Well, from the research and people that I have spoken to LLM is just to let you sit the bar but nothing more. But also I would KEEP IN MIND the quality of school as well. If for some odd reason you end up at Golden Gate University or a tier 4 school then you are just doomed.

But while being a law student(JD, LLB, LLM), one should be doing volunteer work at a non profit law organization. This will give the individual the experience and also network within the legal system.