I have changed the JPEG image quality using the setting provided by the JPEG Image Quality plugin. The WP Retina 2x images generated by this plugin don't seem to respect the setting.
(My intention is to serve all images in high resolution, but highly compressed, by leaving the debug mode on. See Retina Revolution )
By the way, will the Regenerate Thumbnails plugin also regenerate the WP Retina 2x images?