Yes you'll get light fall-off and lower resolution in the corners, since you get closer to the image circle borders. How bad this will be depends on the lens quality. Unfortunately I don't know how sharp the 85mm is, but I know Canon's old 90mm is really sharp - those longer focal lengths are usually a bit sharper than the wider angle T/S lenses.
If your alternative is to turn a panorama head and stitch three images with an unshifted lens you will indeed get rid of the resolution loss and fall-off *per image*, but when you stitch them together and make rectilinear projection out of them (to get the same projection as when using one rectilinear lens with shifting) the images on the side will have to be stretched quite a lot so you will in practice get lower resolution anyway. If you however make a cylindrical projection (usually works very well in landscape, sucks in architecture) there is very little stretch and then you will get better resolution with the panorama head method.
In short, shifting and stitching is better when you need rectilinear projection, but if you can do with cylindrical projection a panorama head is better.
Note that you'll probably want to use your camera in portrait mode when stitching horizontally to get as much resolution as possible (same for panorama of course). The problem that lenses rarely have tripod collars means that you'll need to make sure that the camera body is shifted the corresponding amount in the opposite direction so that the lens stands still. Usually when doing this shifting around there is some small error so you'll need a panorama stitcher to fit the images together, it is rarely as perfect so you can manually fit the images together.
