I always enjoy your photographs and your amazing artistry. My highest compliments!
I basically traded my 10-22mm for the Rokinon 14mm as my kit now includes a 6D. I have a strong interest now in doing nightscapes/Milky Way. My initial testing with the Rokinon seems to show a sizable difference in sharpness between f2.8 and f4 so I'm not sure if I have a good copy of the lens or not. Apart from that, what are your typical exposure settings for nightscapes? Do you usually shoot wide open at f2.8? Focusing in the dark also presents a challenge with this lens, at least for me.
I will often shoot at f/4 because of that extra bit of sharpness, particularly towards the edges. Learn to prefocus your lens. I will often have the focus set before I even go out to shoot at night, and usually pack a flashlight so that I can verify focus (on the distance scale). Don't rely on visually trying to focus the lens at night or you will just be frustrated. Also be aware that on many copies of the lens that infinity focus comes before the hard stop at the end. Don't focus that far!