I have both Mutars, 0,7x and 1,5x. They work quite well if stopped down t o f/8 - with the 75/3,5 Planar lens of my Rolleiflex. I dont see any falloff in the corners. Sharpness is quite good even at edges and corners. I have not tried it with combination of the Rolleicords' Tessar.
It is not necessary to install the 24 exposure kit for that reason. You can use normal film of type 120 and only use a strip of each image.