How economically diversified would the Kingdom of Kongo be in the 15th century - what portion of the population would be involved in specialized jobs, versus how many were farmers? Were farmers primarily subsistence farmers, or operating more in a market? How big were the cities, and how urbanized was the society?
Unfortunately the Christmas holiday put a damper on my efforts to address this question, which is why it's languished. First of all, there is some need for new scholarship on Kongo; it's out there but not quite fully formed yet. We still go back to Thornton's The Kingdom of Kongo and Jan Vansina's Kingdoms of the Savanna for the essentials. There is one even older work in translation from c. 1960, but as you can imagine we must be wary.
The question of urbanites and economies is an interesting one. Certainly Kongo was an unusually well centralized empire, with towns of state owned slaves cultivating for the Manikongo (king) or regional lords. Mbanzakongo (São Salvador) was a major center of course. But the bulk of the population was rural, as was normal for these kingdoms, and the economy of Kongo turned on primary goods with some secondary manufacture (cloth, luxury goods from animal products, etc) but not minerals. The Portuguese were quite unhappy to find that out, and quickly the most valuable things they could get were captives from Kongo's wars. These captives were not always sent to Portuguese or other European run destinations, but often fetched more when sold to societies in West Africa. Warring local rulers, succession crises, and the like all added the demand for captives. Really Kongo was a shell of itself by the late 16th century.
I will add more if I can get my copies of the relevant works. I have ever seen reliable numbers for what you ask, though the towns and cities were smaller than the European port cities, and cultivation was definitely aimed at a surplus.
I would echo /u/Khosikulu's recommendation that John Thornton is the authority to look to regarding the Kingdom of Kongo.
Regarding Kongo urbanism specifically, Dr. Thornton had a very informative chapter titled "Mbanza Kongo/São Salvador Kongo's Holy City" in David Anderson's Africa's Urban Past. The details that follow come directly from that chapter and from “Demography and History in the Kingdom of Kongo, 1550-1750” in the Journal of African History Vol 18, No. 4 (1977)
Mbanza Kongo was founded near the beginning of the 15th century, and by the time of first contact with the Portuguese in 1483 it had grown larger than the other cities in its orbit. King Aphonso came to power in 1506 after a brief civil war, and he did much to change the city both in its architecture and its population. He brought captives taken in the civil war to the capital as slaves. He and his successors would continue this practice throughout the 16th century, drastically increasing the population of the city.
An early Portuguese account in 1491 compares the population of Mbanza Kongo to the Portuguese city of Evora, without further comment. A century later, a more helpful description estimates 10,000 households (perhaps 50,000 people) in what could be called the Mbanza Kongo "metropolitan area". About a decade after that account, in 1604 another credible witness estimates a far smaller population of 2,000 households or perhaps 10,000 people. Thornton explains this discrepancy by suggesting the second figure only included the population within the city walls, or Mbanza Kongo proper, while the earlier larger figure included the entire population of the plateau and valleys surrounding the city, a radius of roughly 10 kilometers.
A separate account in 1610 notes that farming was taking place in this area of heavy settlement surrounding the capital (Thornton estimates settlement density was 10x greater than normal elsewhere in the kingdom outside of cities).
Thorton paints a picture of São Salvador's population steadily growing, estimating a metropolitan population 60,000 residents by 1665, and quoting Giovanni Antonio Cavazzi's period estimate of 70,000 residents of Sao Salvador. Assuming that settlement patterns mirror the example at Mbanza Sonyo, Thornton describes an urban core with a population density of 50 persons per square kilometer, and a periphery with roughly 10 persons per square kilometer.
If Thornton's estimate of 60,000 residents at Sao Salvador in 1665 is correct, that would mean the "metro area" around Mbanza Kongo/São Salvador alone contained about 12 percent of a Kongo kingdom population estimated at 509,000 in 1650^1. Including provincial capitals and other small cities could easily push the urban population past 20 percent of the population.
Of course, all of this has spoken only of the capital so far, and Mbanza Kongo/São Salvador is unusual because of its size. If we look at Thornton's older work Demography and History, he looks at the provincial capital Mbanza Sonyo, and gives a population estimate of 14,600 in 1645. His estimate is arrived at by examining the baptismal records from Capuchin monks conducting missionary activity in the area. Thornton also concludes that Mbanza Sonyo experiences a population explosion and doubles in size to just under 30,000 people by 1700, which matches very well with an estimate by Antonio Zucchelli di Gaschia who lived in that city in 1702-03.
If you have access to JSTOR, here is a stable URL for Demography and History
Also, "Mbanza Kongo/São Salvador Kongo's Holy City" is readable as a google books preview here it is chapter 4, starting on page 67.