Good morning, Aaron;
OK. The use of the simple type adapter such as the Fotodiox will affect the image quality at all distances when used between your Minolta lenses and the Canon EOS-40D body. There will be an additional piece of glass of unknown quality between the lens and the body to correct for losing the ability to focus at infinity due to the thickness of the adapter, and the addition of any glass, especially behind the lens, will have some negative effect on the image quality, just from the fact that it is more glass, even if it is just a plain piece of glass with parallel plano surfaces on both sides of the glass.
One example of this kind of an effect can be found with telephoto lenses where there is provision for using smaller filters in a special holder usually at the back of the telephoto lens. This is to get around the cost of filters in sizes of 77mm, 82mm, 86mm, 95mm, 105mm, 112mm, 122mm, and others for truly impressive lenses. 122mm filters are usually well over $100.00 each. The price for a 30.5mm, 39mm, or 43mm filter for example, is much less, so that is why they chose to do that. But the design of the telephoto lens is a little more critical at that point in the back of the optical path. All of the light rays are bunched together in a much smaller area than at the front of the lens where the light enters the lens. Any glass at this point must be of a good quality with no distortion, or there are effects on the image at the focal plane. The curious thing is that the designer must account for the effect of just the glass in that filter at that point in the lens, and as a result, there must be at least a plain or "normal filter" installed when the lens is used without any of the other colored filters in place. If not, the lens resolution and other qualities are degraded.
An example of this effect is the original Tamron Type 107B SP 300mm f/2.8 LD IF telephoto lens where they forgot to include the rear mounted filter in the optical design, and added the mechanical mount afterward. A true embarrassment. The production samples did not perform well, and in the MODERN PHOTOGRAPHY tests, they found that the lens did much better when they took the "normal filter" out of the lens. The next lens in that series, the Tamron Type 60B, did have the filter included in the optical design, and it performed much better with the filter in place, and the performance dropped when the filter was removed, as would be expected.
OK. So what happens if we follow the suggestion of the Fotodiox people and remove the glass element "for macro-photography?" Right off, we lose the ability to focus at infinity, and also at most of the normal distances we use when taking a photograph. Essentially, we are now using the adapter as a short extension tube for close-up only photography. This means at distances measured in inches, not in feet, with a normal lens.
Well, what does this all settle down to? The Fotodiox adapter will work with your Canon EOS-40D and your Minolta lenses, but you will not get the results you may remember from your Minolta lenses when using them to take photographs on film with a Minolta manual focus camera body. If you are making only small snapshot size prints from the photographs taken with the Minolta-Fotodiox-Canon combination, it probably will not be a problem. If you are planning on 8 by 10, 11 by 14, 16 by 20, and larger, you will see a difference.
Aaron, you are lucky. You have a Canon EOS DSLR camera with an APS-C size digital sensor. The Digital ROKKOR Replacement Lens Mounting Flange will go right onto your camera, and it can be removed and the original Canon EF-S lens mounting flange put back onto the camera without any permanent change. The price of the Digital ROKKOR Lens Mounting Flange is about the same as the Fotodiox adapter, but there is the cost of the shipping from Italy. When I last checked, I think it was about $24.00 for air, but not express. I think it is worth the cost of the shipping to get the better quality and functionality of your Minolta lenses with your Canon EOS-40D DSLR camera.