While I agree that the region that is typically referred to as "Little Dixie" may sometimes be used to reference areas a little further north or south, I don't think that they are accurate descriptions. Yes, Missouri had agriculture, but there were a lot of areas in the north with excellent agriculture that would not fall into a category of "Little Dixie." Many in the counties that fall into the category had emigrated from the upper south (KY, VA, SC, NC, and Tenn.). With them, they brought their ways of life and their slaves. David Hacket Fischer wrote a book titled Albion's Seed. He argued that regions in the United States have distinct cultures because of the origin of their settlers. Specifically, he looked at the settlement of America and noticed that most from VA hailed from the same area of England. Likewise, most in Pennsylvania hailed from the same region. They all brought to America their ways and belief systems. I think that a very good argument can be made that the same is true in Missouri. Those from the upper south that helped settle the areas brought their ways of life. Did that include agriculture? You betcha! Did it include slavery? Unfortunately. Should we be proud of it? No, but we should not run from it either. Trying to create a definition of Little Dixie that does not include a strong foundation of slavery really does not help the discussion. I do, however, think that the region was more than simply a group of slave counties.
I think that you are correct regarding a few of the counties that you considered part of the heart of Little Dixie (Boone, Calloway and Howard). I do wonder, however, where your statistics regarding Clay and Cooper were derived. I used the Library of Virginia's website for census data (http://fisher.lib.virginia.edu/collections/stats/histcensus/). If you have not used it before for research, it is a wonderful way to get a feel for demographics based on the census. You simply click on the year, the data that you would like to see (in this case total slaves), and press "submit query." It will show the data for all of the states. Place a check next to Missouri and at the bottom of the page there is an option for county by county results. It will even map the data for you. With that said, Clay and Cooper, while no slouches in the slave categories do not fall in the top 5. If you map it based on total slaves, the "heart" of Little Dixie jumps out at you.