All the way down to 18 on the 17-55 is going to start to get into the area where diffraction starts to show up, that is why I was asking. What need did you have for a long exposure? At that distance you should be hyperfocal at just about any aperture.
Some days ago, I did a little project. Put the camera on a tripod and shot the same scene with different aperture and focal lengths.
From the 18mm focal length:
The overall sharpest is f11.
Tied for 2nd f8 and f16
The Overall FOV:
I was a bit surprised that the f5.6 was less sharp than the f22... but at the focal end of the lens (18 mm) strange things happen...