If I am not mistaken (please somebody correct me if I am) the way this work is:
A 50mm lens focused at 1m gives you magnification X.
A 100mm lens focused at 2m gives you the same magnification X.
A 50mm at f/2 has a 25mm aperture.
A 100mm at f/4 has a 25mm aperture.
So, a 50mm at f/2 at 1m will give you the same magnification and depth of field as a 100mm at 2m and f/4. The background view will be different due to the different angle of view and obviously the working distance is a practical factor but subject magnification and depth of field should be the same.
A close-up filter simply allows you to focus closer, getting a longer lens that will get you the same magnification from a longer distance will make no difference to depth of field.
If you use a 120 at its min focus of 0.8m and you use a proxar on the 80 to get to the 0.8m then all you are gaining is a bit of practicality and obviously some image sharpness as the 120 is optimised for macro vs the 80 with a random piece of glass in front of it.
In general, things get very blurry outside the focus plane once you get to 1:4 or closer.