Johan Fourie and Jan Greyling have published a paper that calculates wheat productivity in the Cape Colony in 1825. They can do so because of the full transcription of all tax censuses for that year. Although they find some variation across districts, they find much bigger variation within districts. Khoe labour seems more important to explain variations in productivity than slave labour. The paper can be found here: https://www.tandfonline.com/doi/full/10.1080/03031853.2023.2176895