I took this image of M83 back in March 2015. It's only 3 subs, at 120 sec each using ISO 6400, same camera. It also has a generic mid level curve applied to bring out the background noise.
In this image the background noise is random, no smear and no distinct pattern. Unfortunately I have to say at the moment that this final result looks significantly better than my most recent attempt using 5min subs at ISO 800.
This shouldn't be true... Hence the confusion
Cheers, Evan