I followed your suggestion and carried out some basis testing, shooting a test chart at different apertures and found the following results.
At F2.8 sharpness is disappointing. It takes F8 for optimal performance (sharpness and micro contrast) while F5.6 is very satisfactory, being almost equal to F8. At F4 the image is blurred. I will repeat the test to be certain of that specific aperture and to expose at small apertures .
I used a Slik Pro 500DX tripod at its lowest position to prevent vibration, at my P67II I used mirror lock-up. Film was FP4+, semi stand developed in Rodinal 1+100 for 1 hour.
My real-life testing with this glass. All handheld @ F/2.8 and F/4 - Tri-X 400