A highly specific kind of shooting, to be sure. Someone will no doubt have fun with the lens-cap-on part. But nevertheless, the difference is very noticable. Here is an interesting quick view:
http://petapixel.com/2012/12/13/canon-6d-and-5dmk3-noise-comparison-for-high-iso-long-exposures/
http://petapixel.com/2012/12/13/canon-6d-and-5dmk3-noise-comparison-for-high-iso-long-exposures/