18 mm with an 80 mm lens would reduce the maximum f-stop of 2.8 to an effective one of 3.5 (i.e. about 0.5 stop less) with the lens itself set to infinity.

You will want the 18 mm to add to the extension built-in to the focussing mount, so the lens will be set to the close focus limit (what's that, 0.9 m/3 ft?).
Then the reduction is about 0.7 - 0.8 stops, turning f/2.8 into an effective f-stop of just a bit wider than f/4 (at an image scale of about 1:3).