I've used a company locally in Portland called Crossroads Mobile Canning. You bring him a corny or sanke, whatever size you want, he runs a little bit through the lines, and fills 2 cans at a time. Then one can at a time on the sealer. We did 40-45 cans in about 15-20 minutes for 12oz. Another time I did 16oz cans and it went even faster. The loss was negligible in my opinion, no more so than bottling.
He does it for breweries and sometimes will have canning days with our local club. One time I went to him down at his house and we canned there. He loads it on a liftgate truck and travels a bunch. Pretty sweet setup.
Did it for my wedding beer (Raspberry ale), and another time for a "barrel" aged vanilla porter I did.