To look at data from one area, Britain, we have wheat yields of:

Year	1000kg/hectare
1300	0.53
1400	0.42
1600	0.74
1700	0.94
1800	1.25
1860	1.96
2017	7.9

Data up to 1860 from Broadberry, S., Campbell, B., Klein, A., Overton, M., & Van Leeuwen, B. (2015). British Economic Growth, 1270–1870. Cambridge: Cambridge University Press.

A similar result can be seen in Whitney, Milton. “The Yield of Wheat in England during Seven Centuries.” Science, vol. 58, no. 1504, 1923, pp. 320–324, https://www.jstor.org/stable/1648518 - Whitney gives the pre-modern average as about 0.9t/ha (6 bushels/acre).

These yields are averages over both good and bad land, over both good and bad years. The yield in a good year would be higher; the yield is a bad year might mean little grain left for eating after seed is kept for the next sowing. For an idea of the harvest-to-harvest variation in yield that is possible, see the first graph in http://www.cropyields.ac.uk/project.php

Yields are dependent on weather and climate. In many places, wheat is grown on rather marginal land, and average yields can be much lower than the quite high 7.9t/ha for Britain above. For example, the average Australian wheat yield is about 1.74 t/ha (while about 8t/ha is achievable in Australia with good land and irrigation). In some places, modern yields are below 1t/ha (where the land is poor, no/little fertilizer is used, and the crops depends on (not very good) rainfall); this is a yield similar to pre-modern yields. These low yields can be commercially viable due to enormously reduced labour costs due to mechanisation; they often represent a spread of wheat-growing into areas made viable due to improved productivity. Other modern low yields are due to the retention of essentially Medieval agricultural technology.

(Your 7800 loaves/ha is probably an equivalent for 7800kg/ha = 7.8t/ha.)