Some recent data on the forces generated by braking devices is presented in the 5th edition of "Mountaineering: The Freedom of the Hills", pp. 131-135.
Figure 7-27 shows the braking force generated as a function of the force applied by the braking hand. A range of values is indicated for the figure 8 device since the force generated depends on the exact shape and finish.
The curves are quite close to lines. Fits for these lines yield:
(where HF is the force applied by the braking hand, all forces in Newtons)
The graph gives data only from 222 N HF upwards; the authors assumed that any person could generate this force by holding on to the rope.
Page 124 of the same book notes that the friction generated by the Munther hitch is much less dependent on the angle of the rope exiting the knot than is the friction generated by other devices, where there is a more pronounced lock off angle.