Well, I just had to try it.  On 8/14/05, I attached a telecompressor to the 500mm, F5.6 mirror lens I recently started experimenting with.  This converted it to a 350mm, F/4 lens.  I picked the Andromeda Galaxy as a test subject and took about a dozen 130 second images and 3 dark frames with this combination.  I also took a longer, 260 second exposure and a matching separate dark frame.  Of course, this object could use 10 times the exposure to bring out all it's glory, but I was just playing around to see how well the telecompressor worked. 

 I used IRIS  to stack the images and clean up noise and a light pollution gradient.  After the preliminary processing, I used Photoshop Elements to color balance and crop the image. 

This lens combination on my Canon Digital Rebel is equivalent to a 560mm F/4, 35mm telephoto lens.  As I feared, the lens still suffers from distortion the further you get from the center of the frame.  I can minimize it by cropping, but distortion is still evident even in the cropped image.