I just took some pictures to help show the difference.
Incandescent running lights only:
IMG_20171130_160731 by
botond.pal, on Flickr
Incandescent running lights with LED brake lights:
IMG_20171130_160846 by
botond.pal, on Flickr
LED running lights on the left, incandescent running lights on the right (left trunk light was swapped but caught it on the odd refresh rate so it doesn't show properly in the pic):
IMG_20171130_161048 by
botond.pal, on Flickr
LED brake lights on both sides, left has LED running lights and right has incandescent running lights:
IMG_20171130_161122 by
botond.pal, on Flickr
As you can see from the last pic, the running light is almost as bright as the brake on application.